diff --git a/CHANGELOG.md b/CHANGELOG.md
index bd4c1c2..16070c5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -214,6 +214,87 @@ Design: `docs/design/swmr.md`.
   that hangs up, 16 threads on one remote file, and a thread that keeps
   running while a read waits on 0.2 s requests.
 
+### Range reads, milestone M4: remote files in the browser (2026-09-27)
+- **`openUrl(url, opts)`** in `clawhdf5-wasm` opens an HDF5/NetCDF-4 file
+  on a web server without downloading it: the returned `RemoteFile` has
+  `H5File`'s methods (`kind`, `list`, `info`, `attrs`, `attrErrors`,
+  `read`, `readHyperslab`), each returning a promise, and `stats()`
+  (requests, bytes fetched, file size). Only the byte ranges a call needs
+  are fetched, with `fetch` and `Range` headers, in 1 MiB blocks
+  (`blockSize`) kept in a 64 MiB cache (`cacheSize`). `open(bytes)` is
+  unchanged.
+- **How, on the browser's main thread:** the design's restartable
+  "NeedBytes" mode (`clawhdf5_wasm::lazy::LazyStorage`). A call runs as a
+  pass over the blocks fetched so far; a read that misses records the
+  missing blocks and fails, the pass's result is dropped (even if a parser
+  caught the error and carried on), the blocks are fetched and the pass is
+  re-run. No Web Worker and no synchronous XHR (what h5wasm's lazy files
+  need). No block is evicted while a call is in flight, so every call ends;
+  the cache is trimmed between calls, raw-data blocks before metadata.
+  `clawhdf5-remote`'s `BlockCache` is not reused: it fetches by blocking,
+  and it evicts, and does not keep large reads, during a read, where a
+  restartable pass needs every block it read to stay until it finishes.
+- **Every answer is checked** (`js/remote.js`): a `206` with exactly the
+  bytes asked for (`Content-Range`, when the page can see it, and the body
+  length), and the same `ETag`/`Last-Modified` and length as at open, or
+  the call fails: never data from another file or offset. A server that
+  answers `200` to a `Range` request is downloaded whole (up to
+  `maxDownload`, 512 MiB) unless `fallback: "error"`. Other options:
+  `headers`, `credentials`, `parallel` (6 requests at a time), `fetch`.
+- **The viewer** (`examples/wasm-viewer`) has a URL box, opens
+  `?file=<url>` lazily, and shows the requests and bytes fetched.
+- Counted on tank, 2026-09-27 (`WASM_BIG_MB=200 bash
+  examples/wasm-viewer/test/run.sh`, requests as `test/serve.py` counted
+  them): in a 200 MB h5py file, listing the root, reading two small
+  datasets, a group's attributes, the large dataset's shape and a
+  10-value window of it (25 million values) took 5 requests and 6 MiB.
+  With
+  `CLAWHDF5_WASM_CORPUS=conformance/.cache/corpus`, the 622 corpus files
+  up to 16 MiB that open read over HTTP as from bytes: the same listings,
+  attributes and values (datasets up to 2^20 values), and an error
+  wherever bytes give one. Natively, `cargo test -p clawhdf5-wasm --test
+  lazy` with the same variable compares 656 files (up to 64 MiB, error
+  messages included, against the facade's range-storage path).
+- `Reader::open_storage` (the wasm crate's core over any `Storage`), and
+  variable-length strings resolve through the file's storage rather than
+  `File::as_bytes`.
+- The package is larger: the facade's `Storage` read path is now
+  reachable from JavaScript (it was compiled out before), and the promise
+  glue and `remote.js` add JavaScript. Not measured for the docs yet (the
+  build machine was shared); the viewer README's size table predates M4.
+- **Hardened after review (2026-09-27):**
+  - Sizes a server or a dataset names are errors, never an abort of the
+    wasm module (which took every open file on the page with it): a read
+    past 2 GiB aborted in the lazy cache, reachable by a hostile server
+    claiming a large file and a 2 GiB heap collection, and `read()` of a
+    256 MiB `u8` dataset aborted widening it to 64 bits. New option
+    `maxFetch` (512 MiB, at most 1 GiB): what one call may fetch, and the
+    longest single read, refused before fetching. `read()` refuses a
+    dataset that would take more than 1 GiB to decode, naming
+    `readHyperslab`. A file of 4 GiB or more is refused at open (wasm32
+    reads offsets as 32-bit); `maxDownload` is at most 1 GiB.
+  - Response bodies are read as they arrive and cut off at the length
+    asked for (`maxDownload` for a `200`): a `206` with a gigabyte body
+    was buffered whole before its length was checked.
+  - Listing a group reads every child's header, and every node of each
+    level of the group's index, in one pass: 3000 datasets (h5py, 198 MB)
+    listed in 6 passes and 73 requests at 1 MiB blocks instead of 185
+    passes and 184 serial requests (`libver="latest"`: 9 passes instead of
+    189). In `clawhdf5-format`, the B-tree v1/v2 collectors, the symbol
+    table node loop and the dense-link loop read (without using) the
+    siblings after the first that fails, then return that error: same
+    results and errors, more reads only on failure (free in memory). The
+    lazy cache no longer re-fetches a cached block to merge two requests.
+  - `headers` may be a `Headers` instance or `[name, value]` pairs (a
+    `Headers` was silently dropped); when one range request fails the
+    others in flight are aborted; `parallel` must be an integer from 1 to
+    1024.
+  - Tests: `test/serve.py` serves ranges without exposed `Content-Range`/
+    `ETag` (`/noexpose/`, and `/unexposed/` for a real cross-origin page
+    in Chromium), so the HEAD-length path runs end to end; hostile and
+    oversized files (`make_fixture.py`'s `write_limits`), flooding
+    bodies, aborted siblings.
+
 ### Range reads, milestone M3: remote files (2026-09-26)
 - **New crate `clawhdf5-remote`.** `open_url("http://host/file.h5")` gives
   a `clawhdf5::File` (through `File::open_storage`) that reads the file by
diff --git a/CLAUDE.md b/CLAUDE.md
index b382f26..8c95cc5 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -181,14 +181,19 @@ Cargo workspace with 19 crates under `crates/` (plus `libaec-sys`, an internal F
   `CLAWHDF5_REMOTE_CORPUS=conformance/.cache/corpus` compares every corpus
   file over HTTP with `File::open`.
 - GPU-accelerated vector distance computation (`clawhdf5-gpu`, wgpu); HDF5 I/O itself is CPU-only
-- Browser: `clawhdf5-wasm` (wasm-bindgen, read-only, file held in memory;
-  no Zstd/SZIP since they link C) and the `examples/wasm-viewer/` page.
-  `examples/wasm-viewer/test/run.sh` builds the package (needs the
-  `wasm-bindgen` CLI at the crate's exact version) and tests it under Node
-  and headless Chromium (a Playwright download in `~/.cache/ms-playwright`
-  on tank); the CI container has neither, so CI runs the native
-  `clawhdf5-wasm` `h5py_interop` test on the same fixture. Size numbers are
-  in the example's README.
+- Browser: `clawhdf5-wasm` (wasm-bindgen, read-only; no Zstd/SZIP since
+  they link C) and the `examples/wasm-viewer/` page. `open(bytes)` holds
+  the file in memory; `openUrl(url)` (range-read M4) reads it by HTTP range
+  requests through the restartable "NeedBytes" cache (`src/lazy.rs`: a
+  call is re-run after each wave of misses; no block evicted while a call
+  runs), the HTTP in `js/remote.js`. `examples/wasm-viewer/test/run.sh`
+  builds the package (needs the `wasm-bindgen` CLI at the crate's exact
+  version) and tests it under Node and headless Chromium (a Playwright
+  download in `~/.cache/ms-playwright` on tank) against `test/serve.py`
+  (range server with request counts, 200 MB budget file); the CI container
+  has neither, so CI runs the native `h5py_interop` and `lazy` tests
+  (`CLAWHDF5_WASM_CORPUS=conformance/.cache/corpus` for the corpus). Size
+  numbers in the example's README predate `openUrl`.
 - Python and Node.js bindings for cross-language use
 - NetCDF-4 compatibility for scientific data interop
 
diff --git a/README.md b/README.md
index 70ced48..348f832 100644
--- a/README.md
+++ b/README.md
@@ -827,7 +827,7 @@ clawhdf5 workspace (19 crates, ~86K lines of Rust in src/, ~104K with tests
 ├── Bindings
 │   ├── clawhdf5-py          — Python (PyO3)
 │   ├── clawhdf5-napi        — Node.js (napi-rs)
-│   └── clawhdf5-wasm        — Browser (WebAssembly, wasm-bindgen; read-only)
+│   └── clawhdf5-wasm        — Browser (WebAssembly, wasm-bindgen; read-only; remote files by HTTP range requests)
 │
 └── Tooling
     ├── clawhdf5-tools       — h5rs: ls, dump, stat, diff, check
diff --git a/crates/clawhdf5-format/src/btree_v1.rs b/crates/clawhdf5-format/src/btree_v1.rs
index ca4d9d6..ea4e5b1 100644
--- a/crates/clawhdf5-format/src/btree_v1.rs
+++ b/crates/clawhdf5-format/src/btree_v1.rs
@@ -199,19 +199,32 @@ fn collect_symbol_table_nodes_inner<S: Storage + ?Sized>(
         // Leaf: children are SNOD addresses
         Ok(node.children)
     } else {
-        // Internal: recurse into children
+        // Internal: recurse into children. After the first child that
+        // fails, the others are only read (as `storage::touch` does), not
+        // descended into; that error is returned.
         let mut result = Vec::new();
+        let mut failed = None;
         for &child_addr in &node.children {
-            let child_snods = collect_symbol_table_nodes_inner(
+            if failed.is_some() {
+                // Parsing reads the node's header, then its body.
+                let _ = BTreeV1Node::parse_in(file, child_addr, offset_size, length_size);
+                continue;
+            }
+            match collect_symbol_table_nodes_inner(
                 file,
                 child_addr,
                 offset_size,
                 length_size,
                 depth + 1,
-            )?;
-            result.extend(child_snods);
+            ) {
+                Ok(child_snods) => result.extend(child_snods),
+                Err(e) => failed = Some(e),
+            }
+        }
+        match failed {
+            Some(e) => Err(e),
+            None => Ok(result),
         }
-        Ok(result)
     }
 }
 
diff --git a/crates/clawhdf5-format/src/btree_v2.rs b/crates/clawhdf5-format/src/btree_v2.rs
index 5641eb7..f721f40 100644
--- a/crates/clawhdf5-format/src/btree_v2.rs
+++ b/crates/clawhdf5-format/src/btree_v2.rs
@@ -504,45 +504,63 @@ fn collect_internal_records<S: Storage + ?Sized>(
 
     // Interleave: child[0], record[0], child[1], record[1], ..., child[nr]
     // We collect child[0] records, then record[0], then child[1], etc.
+    // After the first child that fails, the others are only touched (see
+    // `storage::touch`); that error is returned.
+    let mut failed = None;
     for (i, &(child_addr, child_nrec)) in node.children.iter().enumerate() {
-        if child_depth == 0 {
-            // Before parsing, so a refused tree is not also a large allocation.
-            spend(budget, usize::from(child_nrec))?;
-            let leaf_recs = parse_leaf_records(
-                file,
-                to_usize(child_addr)?,
-                child_nrec,
-                record_size,
-                node_size,
-            )?;
-            out.extend(leaf_recs);
-        } else {
-            collect_internal_records(
-                file,
-                to_usize(child_addr)?,
-                child_nrec,
-                child_depth,
-                record_size,
-                node_size,
-                offset_size,
-                length_size,
-                max_leaf_nrec,
-                budget,
-                out,
-            )?;
+        if failed.is_some() {
+            let len = usize::try_from(node_size)
+                .unwrap_or(usize::MAX)
+                .min(1 << 16);
+            crate::storage::touch(file, child_addr, len);
+            continue;
         }
+        if let Err(e) = (|| -> Result<(), FormatError> {
+            if child_depth == 0 {
+                // Before parsing, so a refused tree is not also a large allocation.
+                spend(budget, usize::from(child_nrec))?;
+                let leaf_recs = parse_leaf_records(
+                    file,
+                    to_usize(child_addr)?,
+                    child_nrec,
+                    record_size,
+                    node_size,
+                )?;
+                out.extend(leaf_recs);
+            } else {
+                collect_internal_records(
+                    file,
+                    to_usize(child_addr)?,
+                    child_nrec,
+                    child_depth,
+                    record_size,
+                    node_size,
+                    offset_size,
+                    length_size,
+                    max_leaf_nrec,
+                    budget,
+                    out,
+                )?;
+            }
 
-        // Add record[i] (except after the last child)
-        if i < nr {
-            let data = node.record(i, rs)?;
-            spend(budget, 1)?;
-            out.push(BTreeV2Record {
-                data: data.to_vec(),
-            });
+            // Add record[i] (except after the last child)
+            if i < nr {
+                let data = node.record(i, rs)?;
+                spend(budget, 1)?;
+                out.push(BTreeV2Record {
+                    data: data.to_vec(),
+                });
+            }
+            Ok(())
+        })() {
+            failed = Some(e);
         }
     }
 
-    Ok(())
+    match failed {
+        Some(e) => Err(e),
+        None => Ok(()),
+    }
 }
 
 /// The records of a B-tree v2 that fall in one key range, found by
diff --git a/crates/clawhdf5-format/src/group_v1.rs b/crates/clawhdf5-format/src/group_v1.rs
index ebd4d32..bf7dcc4 100644
--- a/crates/clawhdf5-format/src/group_v1.rs
+++ b/crates/clawhdf5-format/src/group_v1.rs
@@ -79,27 +79,51 @@ pub(crate) fn v1_group_entries<S: Storage + ?Sized>(
         length_size,
     )?;
 
+    // The names are read one by one from the heap's data segment; read
+    // (up to 1 MiB of) it first, so a storage that records what it lacks
+    // asks for it at once (see `storage::touch`).
+    if !snod_addrs.is_empty() {
+        let len = usize::try_from(heap.data_segment_size).map_or(1 << 20, |n| n.min(1 << 20));
+        crate::storage::touch(file_data, heap.data_segment_address, len);
+    }
+
     let mut entries = Vec::new();
     let mut heap_checked = false;
+    // After the first node that fails, the others are only read (as
+    // `storage::touch` does); that error is returned.
+    let mut failed = None;
     for snod_addr in snod_addrs {
-        let snod = SymbolTableNode::parse_in(file_data, checked_addr(snod_addr)?, offset_size)?;
-        for entry in &snod.entries {
-            // Like libhdf5, look at the heap's free list only once a name is
-            // needed: an empty group with a damaged heap still lists.
-            if !heap_checked {
-                heap.validate_free_list_in(file_data, length_size)?;
-                heap_checked = true;
+        if failed.is_some() {
+            let _ = SymbolTableNode::parse_in(file_data, snod_addr, offset_size);
+            continue;
+        }
+        let mut node = || -> Result<(), FormatError> {
+            let snod = SymbolTableNode::parse_in(file_data, checked_addr(snod_addr)?, offset_size)?;
+            for entry in &snod.entries {
+                // Like libhdf5, look at the heap's free list only once a name
+                // is needed: an empty group with a damaged heap still lists.
+                if !heap_checked {
+                    heap.validate_free_list_in(file_data, length_size)?;
+                    heap_checked = true;
+                }
+                let name = heap.read_string_in(file_data, entry.link_name_offset)?;
+                entries.push(GroupEntry {
+                    name,
+                    object_header_address: entry.object_header_address,
+                    cache_type: entry.cache_type,
+                });
             }
-            let name = heap.read_string_in(file_data, entry.link_name_offset)?;
-            entries.push(GroupEntry {
-                name,
-                object_header_address: entry.object_header_address,
-                cache_type: entry.cache_type,
-            });
+            Ok(())
+        };
+        if let Err(e) = node() {
+            failed = Some(e);
         }
     }
 
-    Ok(entries)
+    match failed {
+        Some(e) => Err(e),
+        None => Ok(entries),
+    }
 }
 
 /// Symbol table cache type for a soft link: the scratch pad's first four bytes
diff --git a/crates/clawhdf5-format/src/group_v2.rs b/crates/clawhdf5-format/src/group_v2.rs
index dbc1f08..a3e026d 100644
--- a/crates/clawhdf5-format/src/group_v2.rs
+++ b/crates/clawhdf5-format/src/group_v2.rs
@@ -126,6 +126,9 @@ fn for_each_dense_link<S: Storage + ?Sized>(
     )?;
     let records = collect_btree_v2_records_in(file_data, &btree_hdr, offset_size, length_size)?;
 
+    // After the first link that fails, the others are only read, not
+    // visited (a touch, see `storage::touch`); that error is returned.
+    let mut failed = None;
     for record in &records {
         // For type 5 (name index): hash(4) + heap_id(heap_id_length)
         // For type 6 (creation order): creation_order(8) + heap_id(heap_id_length)
@@ -141,12 +144,20 @@ fn for_each_dense_link<S: Storage + ?Sized>(
         let id_bytes = &record.data[id_offset..id_offset + fh.heap_id_length as usize];
 
         // Read managed object from fractal heap
-        let link_data = fh.read_managed_object_in(file_data, id_bytes, offset_size)?;
-        if let Some(link) = parse_link(&link_data, offset_size)? {
-            visit(link);
+        let link_data = fh.read_managed_object_in(file_data, id_bytes, offset_size);
+        if failed.is_some() {
+            continue;
+        }
+        match link_data.and_then(|d| parse_link(&d, offset_size)) {
+            Ok(Some(link)) => visit(link),
+            Ok(None) => {}
+            Err(e) => failed = Some(e),
         }
     }
-    Ok(())
+    match failed {
+        Some(e) => Err(e),
+        None => Ok(()),
+    }
 }
 
 /// Resolve entries from dense storage (fractal heap + B-tree v2).
diff --git a/crates/clawhdf5-format/src/storage.rs b/crates/clawhdf5-format/src/storage.rs
index d53bdc7..5fe9497 100644
--- a/crates/clawhdf5-format/src/storage.rs
+++ b/crates/clawhdf5-format/src/storage.rs
@@ -202,6 +202,19 @@ pub(crate) fn len_usize<S: Storage + ?Sized>(file: &S) -> usize {
     usize::try_from(file.len()).unwrap_or(usize::MAX)
 }
 
+/// Read `len` bytes at `offset` and drop them, ignoring any error.
+///
+/// For a traversal that has failed on one sibling (a B-tree child, a
+/// symbol table node, a heap object) and would stop there: it first
+/// touches the siblings it did not get to, so a storage that records what
+/// it lacks — the browser's restartable reader, which fetches over the
+/// network between attempts — learns about all of them in one attempt
+/// instead of one per attempt. Results and errors are unchanged (the first
+/// error is still the one returned); an in-memory read is free.
+pub fn touch<S: Storage + ?Sized>(file: &S, offset: u64, len: usize) {
+    let _ = file.read_at(offset, len);
+}
+
 /// Bytes `[offset, offset + len)`, all of them.
 ///
 /// A range that runs past the end of the storage is
diff --git a/crates/clawhdf5-wasm/Cargo.toml b/crates/clawhdf5-wasm/Cargo.toml
index 303837a..6d222bc 100644
--- a/crates/clawhdf5-wasm/Cargo.toml
+++ b/crates/clawhdf5-wasm/Cargo.toml
@@ -24,6 +24,8 @@ clawhdf5-format = { path = "../clawhdf5-format", version = "2.7.0" }
 # Must match the wasm-bindgen CLI exactly; build.sh checks.
 wasm-bindgen = "0.2.129"
 js-sys = "0.3.106"
+# Promises for openUrl and RemoteFile (pure Rust over js-sys).
+wasm-bindgen-futures = "0.4.79"
 
 [dev-dependencies]
 serde_json = "1"
diff --git a/crates/clawhdf5-wasm/js/remote.js b/crates/clawhdf5-wasm/js/remote.js
new file mode 100644
index 0000000..cbe3990
--- /dev/null
+++ b/crates/clawhdf5-wasm/js/remote.js
@@ -0,0 +1,235 @@
+// HTTP for clawhdf5-wasm's openUrl (see src/lib.rs and src/lazy.rs).
+//
+// The Rust side decides which byte ranges a read needs; this file fetches
+// them with `fetch` and `Range` headers and checks every answer, so a server
+// that ignores the range, answers with other bytes, or serves a file that
+// changed since it was opened is an error, never data. wasm-bindgen copies
+// it into the package (pkg/snippets/...).
+
+const DEFAULT_MAX_DOWNLOAD = 512 * 1024 * 1024;
+const DEFAULT_PARALLEL = 6;
+
+function fetcher(opts) {
+  const f = opts?.fetch ?? globalThis.fetch;
+  if (typeof f !== "function") {
+    throw new Error("openUrl: no fetch() in this environment (pass opts.fetch)");
+  }
+  return f;
+}
+
+// The caller's headers (`opts.headers`: a Headers, [name, value] pairs or a
+// plain object, as fetch takes them; names come out in lower case) with
+// `extra` over them. A Range of the caller's is dropped: this file asks for
+// the ranges.
+function init(opts, extra, method = "GET", signal = undefined) {
+  const headers = {};
+  if (opts?.headers != null) {
+    for (const [name, value] of new Headers(opts.headers)) {
+      if (name !== "range") headers[name] = value;
+    }
+  }
+  Object.assign(headers, extra);
+  return { method, headers, credentials: opts?.credentials, signal };
+}
+
+// `opts.parallel`: range requests in flight at once.
+function parallelism(opts) {
+  const p = opts?.parallel ?? DEFAULT_PARALLEL;
+  if (!Number.isSafeInteger(p) || p < 1) {
+    throw new Error(`openUrl: parallel must be a positive integer, got ${String(p)}`);
+  }
+  return p;
+}
+
+// "bytes a-b/total" -> { start, end (exclusive), total | null }; null when
+// the page cannot see the header (cross-origin, not exposed).
+function contentRange(resp, url) {
+  const v = resp.headers.get("Content-Range");
+  if (v === null) return null;
+  const m = /^bytes (\d+)-(\d+)\/(\d+|\*)$/.exec(v.trim());
+  if (!m) throw new Error(`${url}: the server sent an unusable Content-Range: ${v}`);
+  return { start: Number(m[1]), end: Number(m[2]) + 1, total: m[3] === "*" ? null : Number(m[3]) };
+}
+
+// What pins the file: its ETag, else its Last-Modified (null if neither is
+// visible to this page).
+function validatorOf(resp) {
+  return resp.headers.get("ETag") ?? resp.headers.get("Last-Modified");
+}
+
+async function discard(resp) {
+  try {
+    await resp.body?.cancel();
+  } catch {
+    // Nothing to release.
+  }
+}
+
+// The body, refusing more than `limit` bytes as they arrive: it is piped
+// through a TransformStream that errors the moment the count passes the
+// limit, which cancels the body and so aborts the request. Whatever the
+// server declares or sends, the page never holds more than `limit` bytes of
+// it. (`tooBig(n)` makes the error; n is the count so far.) A reader loop
+// would do the same, but it stalls on small bodies in headless Chromium
+// under --virtual-time-budget, which the page test uses; a pipe does not.
+async function readCapped(resp, limit, tooBig) {
+  const declared = resp.headers.get("Content-Length");
+  if (declared !== null && Number(declared) > limit) {
+    await discard(resp);
+    throw tooBig(declared);
+  }
+  if (!resp.body) {
+    // No stream to read from (some fetch implementations): all at once.
+    const all = new Uint8Array(await resp.arrayBuffer());
+    if (all.length > limit) throw tooBig(all.length);
+    return all;
+  }
+  let n = 0;
+  let over = null;
+  const capped = resp.body.pipeThrough(new TransformStream({
+    transform(chunk, ctl) {
+      n += chunk.length;
+      if (n > limit) {
+        over = tooBig(`over ${limit}`);
+        ctl.error(over);
+        return;
+      }
+      ctl.enqueue(chunk);
+    },
+  }));
+  try {
+    return new Uint8Array(await new Response(capped).arrayBuffer());
+  } catch (e) {
+    throw over ?? e;
+  }
+}
+
+// The whole body of a 200 answer (a server without range support), at
+// most `limit` (maxDownload) bytes.
+function readAll(resp, limit, url) {
+  return readCapped(resp, limit, (n) =>
+    new Error(`${url} is ${n} bytes, more than maxDownload (${limit}); ` +
+      "the server does not support range requests, so the whole file would have to be downloaded"));
+}
+
+// The body of a 206 answer, which may not be longer than the `limit` bytes
+// asked for at `start` (the caller checks the exact length).
+function readLimited(resp, limit, url, start) {
+  return readCapped(resp, limit, () =>
+    new Error(`${url}: asked for ${limit} bytes at offset ${start}, the server sent more`));
+}
+
+/**
+ * Ask for the file's first `firstLen` bytes. A server that honours the
+ * range (206) gives `{ length, first, validator, requests }`; one that
+ * answers 200 sends the whole file, which is kept (`{ whole, requests }`)
+ * when `opts.fallback` is "download" (the default) and the file is at most
+ * `opts.maxDownload` bytes, and is an error otherwise.
+ */
+export async function probe(url, firstLen, opts) {
+  const f = fetcher(opts);
+  parallelism(opts);
+  const resp = await f(url, init(opts, { Range: `bytes=0-${firstLen - 1}` }));
+  if (resp.status === 206) {
+    const cr = contentRange(resp, url);
+    if (cr && cr.start !== 0) {
+      await discard(resp);
+      throw new Error(`${url}: asked for bytes from 0, the server sent bytes from ${cr.start}`);
+    }
+    const first = await readLimited(resp, firstLen, url, 0);
+    let length = cr?.total ?? null;
+    let requests = 1;
+    if (length === null) {
+      // Content-Range is not readable here: a cross-origin server that does
+      // not list it in Access-Control-Expose-Headers. Content-Length of a
+      // HEAD request is always readable.
+      const head = await f(url, init(opts, {}, "HEAD"));
+      requests++;
+      const cl = head.headers.get("Content-Length");
+      if (!head.ok || cl === null) {
+        throw new Error(`${url}: cannot learn the file's size (a cross-origin server must send ` +
+          "Access-Control-Expose-Headers: Content-Range, or answer HEAD with Content-Length)");
+      }
+      length = Number(cl);
+    }
+    if (!Number.isSafeInteger(length) || length < 0) {
+      throw new Error(`${url}: the server gave a file size of ${length} bytes; openUrl reads files ` +
+        "of up to 2^53 - 1 bytes (the largest offset a JavaScript number holds exactly)");
+    }
+    if (first.length !== Math.min(firstLen, length)) {
+      throw new Error(`${url}: asked for the first ${firstLen} bytes of ${length}, got ${first.length}`);
+    }
+    return { length, first, validator: validatorOf(resp), requests };
+  }
+  if (resp.status === 200) {
+    if ((opts?.fallback ?? "download") !== "download") {
+      await discard(resp);
+      throw new Error(`${url}: the server does not support HTTP range requests (it answered 200 ` +
+        "to a Range request); open it with { fallback: \"download\" } to download the whole file");
+    }
+    const whole = await readAll(resp, opts?.maxDownload ?? DEFAULT_MAX_DOWNLOAD, url);
+    return { whole, requests: 1 };
+  }
+  await discard(resp);
+  throw new Error(`${url}: HTTP ${resp.status} ${resp.statusText ?? ""}`.trim());
+}
+
+/**
+ * Fetch `ranges` ([start0, end0, start1, end1, ...], ends exclusive) of a
+ * file opened by `probe`, at most `opts.parallel` (default 6) at a time.
+ * Every answer must be a 206 with exactly the bytes asked for, from the same
+ * file (validator and length). When one request fails, the others in
+ * flight are aborted and no more are made; that failure is the error.
+ */
+export async function fetchRanges(url, ranges, opts, validator, length) {
+  const f = fetcher(opts);
+  const parallel = parallelism(opts);
+  const n = ranges.length / 2;
+  const out = new Array(n);
+  const abort = new AbortController();
+  let next = 0;
+  async function one(i) {
+    const start = ranges[2 * i];
+    const end = ranges[2 * i + 1];
+    const resp = await f(url, init(opts, { Range: `bytes=${start}-${end - 1}` }, "GET", abort.signal));
+    if (resp.status !== 206) {
+      await discard(resp);
+      throw new Error(resp.status === 200
+        ? `${url}: the server stopped honouring range requests`
+        : `${url}: HTTP ${resp.status} ${resp.statusText ?? ""}`.trim());
+    }
+    const cr = contentRange(resp, url);
+    const v = validatorOf(resp);
+    if ((validator != null && v !== null && v !== validator) ||
+        (cr?.total != null && cr.total !== length)) {
+      await discard(resp);
+      throw new Error(`${url} changed on the server since it was opened`);
+    }
+    if (cr && (cr.start !== start || cr.end !== end)) {
+      await discard(resp);
+      throw new Error(`${url}: asked for bytes ${start}-${end - 1}, the server sent ${cr.start}-${cr.end - 1}`);
+    }
+    const body = await readLimited(resp, end - start, url, start);
+    if (body.length !== end - start) {
+      throw new Error(`${url}: asked for ${end - start} bytes at offset ${start}, got ${body.length}`);
+    }
+    out[i] = body;
+  }
+  async function worker() {
+    while (next < n && !abort.signal.aborted) {
+      try {
+        await one(next++);
+      } catch (e) {
+        // The first failure stops the rest: requests in flight are aborted
+        // (their AbortErrors are not reported) and no new ones start.
+        if (!abort.signal.aborted) {
+          abort.abort();
+          throw e;
+        }
+        return;
+      }
+    }
+  }
+  await Promise.all(Array.from({ length: Math.min(parallel, n) }, worker));
+  return out;
+}
diff --git a/crates/clawhdf5-wasm/src/core.rs b/crates/clawhdf5-wasm/src/core.rs
index 4c6beab..0b5f78e 100644
--- a/crates/clawhdf5-wasm/src/core.rs
+++ b/crates/clawhdf5-wasm/src/core.rs
@@ -6,11 +6,25 @@
 //! with no typed-array mapping (compound, reference, opaque, ...) is refused
 //! with a message naming it, never returned as reinterpreted bytes.
 
+use std::sync::Arc;
+
 use clawhdf5::{AttrValue, File, Selection};
 use clawhdf5_format::data_read;
 use clawhdf5_format::datatype::{Datatype, DatatypeByteOrder};
+use clawhdf5_format::message_type::MessageType;
+use clawhdf5_format::object_header::ObjectHeader;
+use clawhdf5_format::storage::Storage;
 use clawhdf5_format::vl_data::{VlResolver, check_element_size};
 
+/// The most memory one read may use while it decodes: the stored bytes,
+/// the values at 64 bits (integers are widened first) and the values
+/// returned. A larger read fails with an error naming `readHyperslab`,
+/// before anything is read: on wasm32 a buffer past 2 GiB cannot be
+/// allocated at all, and failing to allocate aborts the module (every open
+/// file on the page with it). 1 GiB leaves room in wasm32's 4 GiB for the
+/// file's cached blocks and the JavaScript copy of the result.
+pub const MAX_READ_BYTES: u64 = 1 << 30;
+
 /// Errors are reported to JavaScript as messages.
 pub type Result<T> = std::result::Result<T, String>;
 
@@ -119,7 +133,9 @@ pub struct Attr {
     pub value: AttrValue,
 }
 
-/// An open file, held in memory.
+/// An open file: held in memory ([`Reader::open`]) or read through a
+/// [`Storage`] ([`Reader::open_storage`], such as a
+/// [`LazyStorage`](crate::lazy::LazyStorage)).
 pub struct Reader {
     file: File,
 }
@@ -132,6 +148,14 @@ impl Reader {
         })
     }
 
+    /// Open a file read through `storage` (the file's bytes from offset 0,
+    /// user block included, as [`File::open_storage`] takes them).
+    pub fn open_storage(storage: Arc<dyn Storage + Send + Sync>) -> Result<Self> {
+        Ok(Self {
+            file: File::open_storage(storage).map_err(err)?,
+        })
+    }
+
     /// Whether `path` names a group or a dataset.
     pub fn kind(&self, path: &str) -> Result<Kind> {
         match self.file.dataset(path) {
@@ -144,31 +168,54 @@ impl Reader {
     /// The groups, then the datasets, in the group at `path` (`/` is the
     /// root). Soft links are listed as their targets; external and dangling
     /// links, and named datatypes, are left out.
+    ///
+    /// What [`Group::groups`](clawhdf5::Group::groups) and `datasets` list,
+    /// but every child's object header is read before an error ends the
+    /// listing (the first error, in listing order, is the one returned, as
+    /// there). Over a [`LazyStorage`](crate::lazy::LazyStorage) that makes
+    /// one pass ask for all the headers it is missing at once, instead of
+    /// one pass, and one round trip, per header.
     pub fn list(&self, path: &str) -> Result<Vec<Child>> {
         if self.kind(path)? != Kind::Group {
             return Err(format!("not a group: {path}"));
         }
         let group = self.file.group(path).map_err(err)?;
-        let mut out: Vec<Child> = group
-            .groups()
-            .map_err(err)?
-            .into_iter()
-            .map(|name| Child {
-                name,
-                kind: Kind::Group,
-            })
-            .collect();
-        out.extend(
-            group
-                .datasets()
-                .map_err(err)?
-                .into_iter()
-                .map(|name| Child {
-                    name,
-                    kind: Kind::Dataset,
-                }),
-        );
-        Ok(out)
+        let entries = group.entries().map_err(err)?;
+        let sb = self.file.superblock();
+        let storage = self.file.storage();
+        let mut groups = Vec::new();
+        let mut datasets = Vec::new();
+        let mut first_error = None;
+        for (name, address) in entries {
+            match ObjectHeader::parse_in(storage, address, sb.offset_size, sb.length_size) {
+                Ok(header) => {
+                    let has = |t: MessageType| header.messages.iter().any(|m| m.msg_type == t);
+                    if has(MessageType::LinkInfo)
+                        || has(MessageType::Link)
+                        || has(MessageType::SymbolTable)
+                    {
+                        groups.push(Child {
+                            name: name.clone(),
+                            kind: Kind::Group,
+                        });
+                    }
+                    if has(MessageType::DataLayout) {
+                        datasets.push(Child {
+                            name,
+                            kind: Kind::Dataset,
+                        });
+                    }
+                }
+                Err(e) => {
+                    first_error.get_or_insert(e);
+                }
+            }
+        }
+        if let Some(e) = first_error {
+            return Err(err(clawhdf5::Error::from(e)));
+        }
+        groups.extend(datasets);
+        Ok(groups)
     }
 
     /// Shape, max shape and datatype of the dataset at `path`.
@@ -228,13 +275,27 @@ impl Reader {
         if let Datatype::VariableLength { size, .. } = array_base(&dt) {
             check_element_size(*size, self.file.superblock().offset_size).map_err(err)?;
         }
-        let raw = ds.read_selection(&selection).map_err(err)?;
-        let data = self.decode(&raw, &dt)?;
         out_shape.extend(element_shape(&dt));
         let expected = out_shape
             .iter()
             .try_fold(1u64, |acc, &d| acc.checked_mul(d))
             .ok_or("selection size overflows")?;
+        let cost = expected.saturating_mul(bytes_per_value(&dt));
+        if cost > MAX_READ_BYTES {
+            return Err(format!(
+                "reading {path}{} would take about {} MiB of memory, more than the {} MiB \
+                 one read may use; read it in parts (readHyperslab)",
+                if slab.is_some() {
+                    " (this selection)"
+                } else {
+                    " whole"
+                },
+                cost >> 20,
+                MAX_READ_BYTES >> 20
+            ));
+        }
+        let raw = ds.read_selection(&selection).map_err(err)?;
+        let data = self.decode(&raw, &dt)?;
         if data.len() as u64 != expected {
             return Err(format!(
                 "read {} values for shape {out_shape:?} ({expected} expected)",
@@ -281,11 +342,14 @@ impl Reader {
                 // string ends at its first NUL and a heap object of the
                 // wrong size is an error, as in libhdf5 and h5py.
                 let sb = self.file.superblock();
-                Data::Strings(
-                    VlResolver::new(self.file.as_bytes(), sb.offset_size, sb.length_size)
-                        .strings(raw)
-                        .map_err(err)?,
-                )
+                let strings = match self.file.contiguous_bytes() {
+                    Some(bytes) => {
+                        VlResolver::new(bytes, sb.offset_size, sb.length_size).strings(raw)
+                    }
+                    None => VlResolver::new_in(self.file.storage(), sb.offset_size, sb.length_size)
+                        .strings(raw),
+                };
+                Data::Strings(strings.map_err(err)?)
             }
             Datatype::Enumeration { .. } if !is_array => {
                 Data::Strings(data_read::read_enum_names(raw, dt).map_err(err)?)
@@ -300,6 +364,27 @@ impl Reader {
     }
 }
 
+/// Memory one value of type `dt` takes while [`Reader::read`] decodes it
+/// (an array type's elements count as values): its stored bytes, plus what
+/// [`Reader::decode`] builds from them. A string counts its `String` (24
+/// bytes on 64-bit targets, less on wasm32) and, for a fixed-length one,
+/// its text; a variable-length string's text lives in the heap and is
+/// bounded by the storage's own read limit.
+fn bytes_per_value(dt: &Datatype) -> u64 {
+    let base = array_base(dt);
+    let stored = u64::from(base.type_size());
+    stored
+        + match base {
+            Datatype::FloatingPoint { size, .. } if *size <= 4 => 4,
+            Datatype::FloatingPoint { .. } => 8,
+            // Widened to 64 bits, then narrowed to a new vector.
+            Datatype::FixedPoint { .. } => 8 + stored,
+            Datatype::String { .. } => 24 + stored,
+            Datatype::VariableLength { .. } | Datatype::Enumeration { .. } => 24,
+            _ => 0,
+        }
+}
+
 /// Narrow integers read at 64 bits to the dataset's own width. The source is
 /// that width, so this cannot fail on correct input; it is checked anyway.
 fn narrow<S: Copy + std::fmt::Display, T: TryFrom<S>>(v: Vec<S>) -> Result<Vec<T>> {
diff --git a/crates/clawhdf5-wasm/src/lazy.rs b/crates/clawhdf5-wasm/src/lazy.rs
new file mode 100644
index 0000000..dadb939
--- /dev/null
+++ b/crates/clawhdf5-wasm/src/lazy.rs
@@ -0,0 +1,815 @@
+//! Reading a file that is not all here, when no read may wait for the
+//! network: the restartable "NeedBytes" mode of `docs/design/range-reads.md`
+//! (milestone M4).
+//!
+//! A browser's main thread cannot block on `fetch`, and the parsers are
+//! synchronous. So an operation (open, list a group, read a dataset) runs
+//! as a *pass* over a [`LazyStorage`] that holds the blocks fetched so far:
+//!
+//! 1. [`LazyStorage::attempt`] runs the operation. A read whose blocks are
+//!    all present is served; a read that misses records the missing blocks
+//!    and fails with a storage error.
+//! 2. If the pass missed anything, its result is thrown away — whatever it
+//!    is, since a parser may have caught the error and carried on (a
+//!    listing skips a link it cannot resolve) — and the caller gets the
+//!    byte ranges to fetch ([`Step::Need`]).
+//! 3. The caller fetches them (asynchronously, with HTTP `Range` requests),
+//!    hands them over with [`LazyStorage::supply`] and runs the operation
+//!    again.
+//!
+//! A pass is pure over the storage: the facade only caches what completed
+//! reads decoded (its chunk cache), so re-running it is safe. Every pass
+//! that does not finish asks for at least one block not yet present, and no
+//! block is evicted while an operation is in flight
+//! ([`LazyStorage::operation`]), so an operation finishes after at most one
+//! pass per block it needs. In practice it is one pass per *wave* of
+//! misses: a chunked read asks for all the chunks of a batch at once.
+//!
+//! Blocks are kept in an LRU cache with a byte budget, trimmed only when no
+//! operation is in flight. Blocks fetched for bulk reads (raw data: a
+//! `read_ranges` call, or a read longer than a block) go first, so reading
+//! a large dataset does not evict the metadata.
+
+use std::borrow::Cow;
+use std::collections::{BTreeSet, HashMap};
+use std::ops::Range;
+use std::sync::{Arc, Mutex, MutexGuard};
+
+use clawhdf5_format::error::FormatError;
+use clawhdf5_format::storage::Storage;
+
+/// Default block size: 1 MiB, as `clawhdf5-remote`'s block cache (the size
+/// `docs/design/range-reads.md` §2 measured).
+pub const DEFAULT_BLOCK_SIZE: u64 = 1 << 20;
+
+/// Default of [`LazyConfig::max_fetch`]: 512 MiB, the same as `openUrl`'s
+/// `maxDownload` for a server without range support.
+pub const DEFAULT_MAX_FETCH: u64 = 512 << 20;
+
+/// The message of the error a read that misses returns. It never reaches
+/// the caller of [`LazyStorage::attempt`]: a pass that missed is re-run.
+pub const NEED_BYTES: &str = "bytes not fetched yet (restartable read)";
+
+/// Settings of a [`LazyStorage`].
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct LazyConfig {
+    /// Size of a block in bytes (at least 512); fetches are whole, aligned
+    /// blocks (the file's last block is shorter).
+    pub block_size: u64,
+    /// Byte budget of cached blocks between operations. An operation keeps
+    /// every block it needs until it finishes, whatever the budget.
+    pub capacity: u64,
+    /// Largest single range asked for, in bytes (whole blocks, at least
+    /// one); longer runs are split so they can be fetched in parallel.
+    pub max_request: u64,
+    /// Most bytes one operation may fetch (at least one block), and so the
+    /// longest single read: a read longer than this fails at once, before
+    /// anything is fetched, and so does an operation whose passes would
+    /// fetch more. The file's length comes from the server, so without
+    /// this a hostile file (a heap "collection" claiming 2 GiB) makes the
+    /// reader fetch and hold whatever it names; on wasm32 a buffer past
+    /// 2 GiB cannot even be allocated.
+    pub max_fetch: u64,
+}
+
+impl Default for LazyConfig {
+    fn default() -> Self {
+        LazyConfig {
+            block_size: DEFAULT_BLOCK_SIZE,
+            capacity: 64 << 20,
+            max_request: 8 << 20,
+            max_fetch: DEFAULT_MAX_FETCH,
+        }
+    }
+}
+
+/// What a [`LazyStorage`] has done so far.
+#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
+pub struct LazyStats {
+    /// Passes run by [`LazyStorage::attempt`].
+    pub passes: u64,
+    /// Ranges handed to [`LazyStorage::supply`]: one HTTP request each.
+    pub requests: u64,
+    /// Bytes handed to [`LazyStorage::supply`].
+    pub bytes_fetched: u64,
+    /// Blocks evicted to stay within the budget.
+    pub evictions: u64,
+    /// Bytes cached now.
+    pub cached_bytes: u64,
+}
+
+/// The outcome of one pass.
+#[derive(Debug)]
+pub enum Step<T> {
+    /// The pass read only bytes that were present: its result stands.
+    Done(T),
+    /// The pass missed: fetch these byte ranges (sorted, disjoint, block
+    /// aligned), [`supply`](LazyStorage::supply) them and run it again.
+    Need(Vec<Range<u64>>),
+}
+
+struct Block {
+    data: Arc<[u8]>,
+    /// Eviction order: bulk blocks (`false`) before metadata (`true`),
+    /// then least recently used first.
+    key: (bool, u64),
+}
+
+#[derive(Default)]
+struct State {
+    blocks: HashMap<u64, Block>,
+    /// `(metadata?, tick, block index)`, in eviction order.
+    order: BTreeSet<(bool, u64, u64)>,
+    tick: u64,
+    bytes: u64,
+    /// Blocks the current pass missed, and whether a small read wanted
+    /// them (metadata).
+    missing: HashMap<u64, bool>,
+    /// Blocks a bulk read missed that have not been supplied yet: kept
+    /// as bulk when they arrive.
+    bulk_pending: BTreeSet<u64>,
+    /// Operations in flight: no eviction while any is.
+    active: u32,
+    stats: LazyStats,
+}
+
+/// A [`Storage`] over the blocks of a file fetched so far; a read of
+/// anything else fails and is recorded, so the pass can be re-run once the
+/// bytes arrive. See the [module documentation](self).
+pub struct LazyStorage {
+    len: u64,
+    config: LazyConfig,
+    state: Mutex<State>,
+}
+
+fn lock(m: &Mutex<State>) -> MutexGuard<'_, State> {
+    m.lock().unwrap_or_else(std::sync::PoisonError::into_inner)
+}
+
+/// Keeps an operation's blocks cached until it is dropped; see
+/// [`LazyStorage::operation`].
+pub struct Operation<'a> {
+    storage: &'a LazyStorage,
+    /// Bytes fetched for this operation so far.
+    fetched: std::cell::Cell<u64>,
+}
+
+impl Operation<'_> {
+    /// Count `ranges` against the operation's budget
+    /// ([`LazyConfig::max_fetch`]) before they are fetched: an error, and
+    /// nothing counted, if they would take it past the budget.
+    pub fn charge(&self, ranges: &[Range<u64>]) -> Result<(), String> {
+        let max = self.storage.config.max_fetch;
+        let total = ranges.iter().fold(self.fetched.get(), |n, r| {
+            n.saturating_add(r.end.saturating_sub(r.start))
+        });
+        if total > max {
+            return Err(format!(
+                "this call would fetch more than {max} bytes of the file (the maxFetch limit); \
+                 read less at a time (readHyperslab) or raise maxFetch"
+            ));
+        }
+        self.fetched.set(total);
+        Ok(())
+    }
+}
+
+impl Drop for Operation<'_> {
+    fn drop(&mut self) {
+        let mut st = lock(&self.storage.state);
+        st.active = st.active.saturating_sub(1);
+        if st.active == 0 {
+            self.storage.evict(&mut st);
+        }
+    }
+}
+
+impl LazyStorage {
+    /// An empty cache for a file of `len` bytes.
+    pub fn new(len: u64, mut config: LazyConfig) -> Self {
+        config.block_size = config.block_size.max(512);
+        config.max_request = (config.max_request / config.block_size).max(1) * config.block_size;
+        config.max_fetch = config.max_fetch.max(config.block_size);
+        LazyStorage {
+            len,
+            config,
+            state: Mutex::new(State::default()),
+        }
+    }
+
+    /// The settings in use (after rounding).
+    pub fn config(&self) -> &LazyConfig {
+        &self.config
+    }
+
+    /// Counters since the storage was made.
+    pub fn stats(&self) -> LazyStats {
+        let st = lock(&self.state);
+        LazyStats {
+            cached_bytes: st.bytes,
+            ..st.stats
+        }
+    }
+
+    /// Mark an operation in flight until the guard is dropped: no block is
+    /// evicted meanwhile, so re-running its passes always makes progress.
+    /// Hold it across every pass of one operation.
+    pub fn operation(&self) -> Operation<'_> {
+        lock(&self.state).active += 1;
+        Operation {
+            storage: self,
+            fetched: std::cell::Cell::new(0),
+        }
+    }
+
+    /// Run one pass of `f` over this storage. `Done` when `f` read nothing
+    /// that is missing; otherwise `Need` with the ranges to fetch, and `f`'s
+    /// result is dropped (it may be an error caused by the miss, or a
+    /// result built around one).
+    pub fn attempt<T>(&self, f: impl FnOnce() -> T) -> Step<T> {
+        {
+            let mut st = lock(&self.state);
+            st.missing.clear();
+            st.stats.passes += 1;
+        }
+        let out = f();
+        let missing = std::mem::take(&mut lock(&self.state).missing);
+        if missing.is_empty() {
+            return Step::Done(out);
+        }
+        drop(out);
+        Step::Need(self.runs(missing))
+    }
+
+    /// The bytes of the file at `offset`, fetched for a range a pass asked
+    /// for. `offset` must be block aligned and the bytes whole blocks (the
+    /// last block of the file may be short) inside the file, or this is an
+    /// error and nothing is kept. Blocks already present are left alone.
+    pub fn supply(&self, offset: u64, bytes: &[u8]) -> Result<(), String> {
+        let bs = self.config.block_size;
+        let end = offset
+            .checked_add(bytes.len() as u64)
+            .filter(|&e| e <= self.len)
+            .ok_or_else(|| {
+                format!(
+                    "{} bytes at offset {offset} run past the end of the {}-byte file",
+                    bytes.len(),
+                    self.len
+                )
+            })?;
+        if !offset.is_multiple_of(bs) || (!end.is_multiple_of(bs) && end != self.len) {
+            return Err(format!(
+                "{} bytes at offset {offset} are not whole {bs}-byte blocks",
+                bytes.len()
+            ));
+        }
+        let mut st = lock(&self.state);
+        st.stats.requests += 1;
+        st.stats.bytes_fetched += bytes.len() as u64;
+        let mut start = offset;
+        while start < end {
+            let i = start / bs;
+            let stop = (start + bs).min(end);
+            if !st.blocks.contains_key(&i) {
+                let rel = (start - offset) as usize..(stop - offset) as usize;
+                let metadata = !st.bulk_pending.remove(&i);
+                self.keep(&mut st, i, Arc::from(&bytes[rel]), metadata);
+            }
+            start = stop;
+        }
+        if st.active == 0 {
+            self.evict(&mut st);
+        }
+        Ok(())
+    }
+
+    /// [`supply`](Self::supply) the bytes fetched for `range`, one of the
+    /// ranges a [`Step::Need`] asked for: anything but exactly its length
+    /// (a server that answered with more or less) is an error.
+    pub fn supply_range(&self, range: &Range<u64>, bytes: &[u8]) -> Result<(), String> {
+        let want = range.end.saturating_sub(range.start);
+        if bytes.len() as u64 != want {
+            return Err(format!(
+                "asked for {want} bytes at offset {}, got {}",
+                range.start,
+                bytes.len()
+            ));
+        }
+        self.supply(range.start, bytes)
+    }
+
+    /// Run `f` to completion, fetching what its passes miss with `fetch`
+    /// (a byte range to its bytes). The blocking driver, for native code
+    /// and tests; the browser's is the same loop with an `await` between
+    /// passes.
+    pub fn run_blocking<T>(
+        &self,
+        mut f: impl FnMut() -> T,
+        mut fetch: impl FnMut(Range<u64>) -> Result<Vec<u8>, String>,
+    ) -> Result<T, String> {
+        let op = self.operation();
+        loop {
+            match self.attempt(&mut f) {
+                Step::Done(v) => return Ok(v),
+                Step::Need(ranges) => {
+                    op.charge(&ranges)?;
+                    for r in ranges {
+                        let bytes = fetch(r.clone())?;
+                        self.supply_range(&r, &bytes)?;
+                    }
+                }
+            }
+        }
+    }
+
+    /// Cache block `i`.
+    fn keep(&self, st: &mut State, i: u64, data: Arc<[u8]>, metadata: bool) {
+        st.tick += 1;
+        let key = (metadata, st.tick);
+        st.bytes += <[u8]>::len(&data) as u64;
+        st.order.insert((key.0, key.1, i));
+        if let Some(old) = st.blocks.insert(i, Block { data, key }) {
+            st.order.remove(&(old.key.0, old.key.1, i));
+            st.bytes -= <[u8]>::len(&old.data) as u64;
+        }
+    }
+
+    fn evict(&self, st: &mut State) {
+        while st.bytes > self.config.capacity {
+            let Some((_, _, i)) = st.order.pop_first() else {
+                break;
+            };
+            if let Some(b) = st.blocks.remove(&i) {
+                st.bytes -= <[u8]>::len(&b.data) as u64;
+                st.stats.evictions += 1;
+            }
+        }
+    }
+
+    /// Byte ranges covering the missing blocks: runs of consecutive
+    /// blocks, a one-block hole between two runs filled so they merge
+    /// (unless the hole is cached: it would be fetched again), each at
+    /// most `max_request` long.
+    fn runs(&self, missing: HashMap<u64, bool>) -> Vec<Range<u64>> {
+        let bs = self.config.block_size;
+        let mut wanted: Vec<u64> = missing.keys().copied().collect();
+        wanted.sort_unstable();
+        let mut st = lock(&self.state);
+        // Remember which blocks only bulk reads asked for: they are kept
+        // as bulk once supplied.
+        for (&i, &metadata) in &missing {
+            if metadata {
+                st.bulk_pending.remove(&i);
+            } else {
+                st.bulk_pending.insert(i);
+            }
+        }
+        let per_request = self.config.max_request / bs;
+        let mut runs: Vec<(u64, u64)> = Vec::new();
+        for i in wanted {
+            match runs.last_mut() {
+                Some((first, last))
+                    if (i == *last + 1
+                        || (i == *last + 2 && !st.blocks.contains_key(&(i - 1))))
+                        && i - *first < per_request =>
+                {
+                    *last = i
+                }
+                _ => runs.push((i, i)),
+            }
+        }
+        drop(st);
+        runs.into_iter()
+            .map(|(a, b)| a * bs..((b + 1) * bs).min(self.len))
+            .collect()
+    }
+
+    /// Block indices covering `[offset, offset + len)`, clamped to the file.
+    fn span(&self, offset: u64, len: u64) -> Option<Range<u64>> {
+        let end = offset.saturating_add(len).min(self.len);
+        if offset >= end {
+            return None;
+        }
+        let bs = self.config.block_size;
+        Some(offset / bs..(end - 1) / bs + 1)
+    }
+
+    /// The blocks of `spans` if all are present (touching them), else
+    /// record the missing ones and fail.
+    fn blocks(
+        &self,
+        spans: &[Range<u64>],
+        metadata: bool,
+    ) -> Result<HashMap<u64, Arc<[u8]>>, FormatError> {
+        let mut st = lock(&self.state);
+        let mut have = HashMap::new();
+        let mut missed = false;
+        for span in spans {
+            for i in span.clone() {
+                if have.contains_key(&i) {
+                    continue;
+                }
+                match st.blocks.get(&i) {
+                    Some(b) => {
+                        have.insert(i, b.data.clone());
+                    }
+                    None => {
+                        missed = true;
+                        let m = st.missing.entry(i).or_insert(metadata);
+                        *m |= metadata;
+                    }
+                }
+            }
+        }
+        if missed {
+            return Err(FormatError::Storage(NEED_BYTES.into()));
+        }
+        // Touch: most recently used last; a small read promotes a bulk
+        // block to metadata.
+        for &i in have.keys() {
+            st.tick += 1;
+            let tick = st.tick;
+            let Some(b) = st.blocks.get_mut(&i) else {
+                continue;
+            };
+            let old = b.key;
+            b.key = (old.0 || metadata, tick);
+            let new = b.key;
+            st.order.remove(&(old.0, old.1, i));
+            st.order.insert((new.0, new.1, i));
+        }
+        Ok(have)
+    }
+
+    /// Refuse a read of `n` bytes longer than an operation may fetch
+    /// ([`LazyConfig::max_fetch`]), before its blocks are asked for.
+    fn check_len(&self, n: u64) -> Result<(), FormatError> {
+        let max = self.config.max_fetch;
+        if n > max {
+            return Err(FormatError::Storage(format!(
+                "a read of {n} bytes is more than one call may fetch ({max} bytes, the maxFetch limit)"
+            )));
+        }
+        Ok(())
+    }
+
+    /// The bytes `offset..end` from `blocks`, which hold every block of
+    /// that span. The buffer is reserved fallibly: a length the address
+    /// space cannot hold (past `isize::MAX` on wasm32) is an error, never
+    /// an abort.
+    fn assemble(
+        &self,
+        offset: u64,
+        end: u64,
+        blocks: &HashMap<u64, Arc<[u8]>>,
+    ) -> Result<Vec<u8>, FormatError> {
+        let bs = self.config.block_size;
+        let n = end - offset;
+        let too_long =
+            || FormatError::Storage(format!("cannot hold a read of {n} bytes in memory"));
+        let mut out = Vec::new();
+        out.try_reserve_exact(usize::try_from(n).map_err(|_| too_long())?)
+            .map_err(|_| too_long())?;
+        let mut pos = offset;
+        while pos < end {
+            let i = pos / bs;
+            let block = &blocks[&i];
+            let from = (pos - i * bs) as usize;
+            let to = ((end - i * bs) as usize).min(<[u8]>::len(block));
+            out.extend_from_slice(&block[from..to]);
+            pos = i * bs + to as u64;
+        }
+        Ok(out)
+    }
+}
+
+impl Storage for LazyStorage {
+    fn read_at(&self, offset: u64, len: usize) -> Result<Cow<'_, [u8]>, FormatError> {
+        let Some(span) = self.span(offset, len as u64) else {
+            return Ok(Cow::Owned(Vec::new()));
+        };
+        let end = offset.saturating_add(len as u64).min(self.len);
+        self.check_len(end - offset)?;
+        let metadata = len as u64 <= self.config.block_size;
+        let blocks = self.blocks(std::slice::from_ref(&span), metadata)?;
+        Ok(Cow::Owned(self.assemble(offset, end, &blocks)?))
+    }
+
+    fn len(&self) -> u64 {
+        self.len
+    }
+
+    fn read_ranges(&self, ranges: &[Range<u64>]) -> Result<Vec<Cow<'_, [u8]>>, FormatError> {
+        let mut spans = Vec::with_capacity(ranges.len());
+        let mut total = 0u64;
+        for r in ranges {
+            if r.end < r.start {
+                return Err(FormatError::Storage(
+                    "read range ends before it starts".into(),
+                ));
+            }
+            total = total.saturating_add(r.end.min(self.len).saturating_sub(r.start));
+            spans.extend(self.span(r.start, r.end - r.start));
+        }
+        self.check_len(total)?;
+        let blocks = self.blocks(&spans, false)?;
+        ranges
+            .iter()
+            .map(|r| {
+                let end = r.end.min(self.len);
+                if r.start >= end {
+                    Ok(Cow::Owned(Vec::new()))
+                } else {
+                    self.assemble(r.start, end, &blocks).map(Cow::Owned)
+                }
+            })
+            .collect()
+    }
+}
+
+#[cfg(test)]
+#[allow(clippy::single_range_in_vec_init)]
+mod tests {
+    use super::*;
+
+    fn file(n: usize) -> Vec<u8> {
+        (0..n).map(|i| (i * 7 + i / 251) as u8).collect()
+    }
+
+    fn config(block: u64, capacity: u64) -> LazyConfig {
+        LazyConfig {
+            block_size: block,
+            capacity,
+            max_request: 4 * block,
+            max_fetch: DEFAULT_MAX_FETCH,
+        }
+    }
+
+    /// Supply every range of `need` from `data`.
+    fn serve(s: &LazyStorage, data: &[u8], need: &[Range<u64>]) {
+        for r in need {
+            s.supply_range(r, &data[r.start as usize..r.end as usize])
+                .unwrap();
+        }
+    }
+
+    fn owned(r: Result<Cow<'_, [u8]>, FormatError>) -> Result<Vec<u8>, FormatError> {
+        r.map(Cow::into_owned)
+    }
+
+    #[test]
+    fn a_miss_asks_for_whole_blocks_then_the_rerun_reads_them() {
+        let data = file(10_000);
+        let s = LazyStorage::new(data.len() as u64, config(1024, 1 << 20));
+        let Step::Need(need) = s.attempt(|| owned(s.read_at(1500, 1000))) else {
+            panic!("nothing is cached yet");
+        };
+        assert_eq!(need, vec![1024..3072]);
+        serve(&s, &data, &need);
+        let Step::Done(got) = s.attempt(|| owned(s.read_at(1500, 1000))) else {
+            panic!("the blocks were supplied");
+        };
+        assert_eq!(got.unwrap(), &data[1500..2500]);
+        // Past the end: short, then empty, as for a slice.
+        serve(&s, &data, &[9216..10_000]);
+        let Step::Done(tail) = s.attempt(|| owned(s.read_at(9_990, 100))) else {
+            panic!("the last block was supplied");
+        };
+        assert_eq!(tail.unwrap(), &data[9_990..]);
+        assert!(matches!(
+            s.attempt(|| s.read_at(20_000, 10).map(|c| c.len())),
+            Step::Done(Ok(0))
+        ));
+        let st = s.stats();
+        assert_eq!(
+            (st.passes, st.requests, st.bytes_fetched),
+            (4, 2, 2048 + 784)
+        );
+    }
+
+    #[test]
+    fn a_pass_that_swallowed_the_miss_is_still_rerun() {
+        // A parser that catches the error and returns something anyway
+        // (a listing skipping a link it cannot resolve) must not have its
+        // result used.
+        let data = file(4096);
+        let s = LazyStorage::new(data.len() as u64, config(1024, 1 << 20));
+        let step = s.attempt(|| s.read_at(0, 4).map(|b| b.len()).unwrap_or(0));
+        assert!(
+            matches!(step, Step::Need(ref n) if n == &vec![0..1024]),
+            "{step:?}"
+        );
+    }
+
+    #[test]
+    fn read_ranges_asks_for_every_missing_block_in_one_pass() {
+        let data = file(64 * 1024);
+        let s = LazyStorage::new(data.len() as u64, config(1024, 1 << 20));
+        let ranges = [
+            100..200,
+            5000..5100,
+            5200..5300,
+            30_000..33_000,
+            60_000..60_010,
+        ];
+        let read = || {
+            s.read_ranges(&ranges)
+                .map(|v| v.into_iter().map(Cow::into_owned).collect::<Vec<_>>())
+        };
+        let Step::Need(need) = s.attempt(read) else {
+            panic!("nothing is cached yet");
+        };
+        // 5000..5300 is blocks 4 and 5; 30_000..33_000 is blocks 29..=32.
+        assert_eq!(
+            need,
+            vec![
+                0..1024,
+                4096..6144,
+                29 * 1024..33 * 1024,
+                58 * 1024..59 * 1024
+            ]
+        );
+        serve(&s, &data, &need);
+        let Step::Done(got) = s.attempt(read) else {
+            panic!("one pass fetched everything");
+        };
+        for (r, g) in ranges.iter().zip(&got.unwrap()) {
+            assert_eq!(g, &data[r.start as usize..r.end as usize]);
+        }
+    }
+
+    #[test]
+    fn runs_merge_one_block_holes_and_split_long_runs() {
+        let data = file(32 * 1024);
+        let s = LazyStorage::new(data.len() as u64, config(1024, 1 << 20));
+        // Blocks 0 and 2 (a hole of one: merged), 5..=14 (split in fours).
+        let Step::Need(need) = s.attempt(|| {
+            let _ = s.read_at(0, 10);
+            let _ = s.read_at(2048, 10);
+            s.read_ranges(&[5120..15 * 1024]).map(|_| ())
+        }) else {
+            panic!("nothing is cached yet");
+        };
+        assert_eq!(
+            need,
+            vec![0..3072, 5120..9216, 9216..13_312, 13_312..15_360]
+        );
+    }
+
+    #[test]
+    fn a_cached_hole_is_not_fetched_again() {
+        let data = file(8 * 1024);
+        let s = LazyStorage::new(data.len() as u64, config(1024, 1 << 20));
+        serve(&s, &data, &[1024..2048]);
+        // Blocks 0 and 2 missing, 1 cached: two requests, not 0..3072.
+        let Step::Need(need) = s.attempt(|| {
+            let _ = s.read_at(0, 10);
+            s.read_at(2048, 10).map(|_| ())
+        }) else {
+            panic!("blocks 0 and 2 are missing");
+        };
+        assert_eq!(need, vec![0..1024, 2048..3072]);
+    }
+
+    #[test]
+    fn supply_refuses_what_was_not_asked_for() {
+        let data = file(10_000);
+        let s = LazyStorage::new(data.len() as u64, config(1024, 1 << 20));
+        assert!(s.supply(1, &data[1..1025]).unwrap_err().contains("whole"));
+        assert!(s.supply(0, &data[..1000]).unwrap_err().contains("whole"));
+        assert!(
+            s.supply(9216, &[0u8; 1024])
+                .unwrap_err()
+                .contains("past the end")
+        );
+        for wrong in [&data[..1023], &data[..2048]] {
+            assert!(
+                s.supply_range(&(0..1024), wrong)
+                    .unwrap_err()
+                    .contains("asked for 1024 bytes")
+            );
+        }
+        assert_eq!(s.stats().cached_bytes, 0);
+        // The file's last, short block is whole.
+        s.supply(9216, &data[9216..]).unwrap();
+        assert_eq!(s.stats().cached_bytes, 784);
+    }
+
+    #[test]
+    fn an_operation_keeps_its_blocks_whatever_the_budget() {
+        // A budget of one block, an operation that needs eight: without
+        // the operation guard every pass would evict what the last one
+        // fetched and never finish.
+        let data = file(8 * 1024);
+        let s = LazyStorage::new(data.len() as u64, config(1024, 1024));
+        let got = s
+            .run_blocking(
+                || {
+                    (0..8)
+                        .map(|i| owned(s.read_at(i * 1024 + 3, 10)))
+                        .collect::<Result<Vec<_>, _>>()
+                },
+                |r| Ok(data[r.start as usize..r.end as usize].to_vec()),
+            )
+            .unwrap()
+            .unwrap();
+        for (i, g) in got.iter().enumerate() {
+            assert_eq!(g, &data[i * 1024 + 3..i * 1024 + 13]);
+        }
+        // Trimmed to the budget once the operation is over.
+        let st = s.stats();
+        assert_eq!(st.cached_bytes, 1024);
+        assert_eq!(st.evictions, 7);
+        assert_eq!(st.passes, 9, "one pass per block, then the one that ends");
+    }
+
+    #[test]
+    fn bulk_blocks_are_evicted_before_metadata() {
+        let data = file(16 * 1024);
+        let s = LazyStorage::new(data.len() as u64, config(1024, 4 * 1024));
+        let fetch = |r: Range<u64>| Ok(data[r.start as usize..r.end as usize].to_vec());
+        // Metadata: a small read of block 0.
+        s.run_blocking(|| s.read_at(0, 16).map(|_| ()), fetch)
+            .unwrap()
+            .unwrap();
+        // Bulk: raw data over blocks 4..12, more than the budget.
+        s.run_blocking(|| s.read_ranges(&[4096..12 * 1024]).map(|_| ()), fetch)
+            .unwrap()
+            .unwrap();
+        // The metadata block survived: reading it again is a hit.
+        let before = s.stats();
+        assert!(before.cached_bytes <= 4 * 1024);
+        assert!(matches!(
+            s.attempt(|| s.read_at(0, 16).map(|_| ())),
+            Step::Done(Ok(()))
+        ));
+        assert_eq!(s.stats().requests, before.requests);
+    }
+
+    #[test]
+    fn a_read_longer_than_max_fetch_fails_without_fetching() {
+        // A hostile file names a 2 GiB heap collection in a "file" the
+        // server claims is 1 TiB: the read is refused before any block is
+        // asked for (on wasm32 its buffer could not even be allocated).
+        let s = LazyStorage::new(1 << 40, LazyConfig::default());
+        let step = s.attempt(|| s.read_at(4096, (1usize << 31) + 4096).map(|b| b.len()));
+        match step {
+            Step::Done(Err(e)) => assert!(e.to_string().contains("maxFetch"), "{e}"),
+            other => panic!("expected a refusal, got {other:?}"),
+        }
+        let step = s.attempt(|| s.read_ranges(&[0..(600 << 20)]).map(|v| v.len()));
+        assert!(matches!(step, Step::Done(Err(_))), "{step:?}");
+        assert_eq!(s.stats().requests, 0);
+        // At the limit it is an ordinary miss.
+        let s = LazyStorage::new(1 << 40, config(1024, 1 << 20));
+        let step = s.attempt(|| s.read_at(0, DEFAULT_MAX_FETCH as usize).map(|b| b.len()));
+        assert!(matches!(step, Step::Need(_)), "{step:?}");
+    }
+
+    #[test]
+    fn an_operation_stops_at_its_fetch_budget() {
+        // Many small reads, none over the limit, that together would fetch
+        // more than the budget: the operation fails before fetching past it.
+        let data = file(64 * 1024);
+        let mut c = config(1024, 1 << 20);
+        c.max_fetch = 8 * 1024;
+        let s = LazyStorage::new(data.len() as u64, c);
+        let e = s
+            .run_blocking(
+                || {
+                    (0..64)
+                        .map(|i| owned(s.read_at(i * 1024, 8)))
+                        .collect::<Result<Vec<_>, _>>()
+                },
+                |r| Ok(data[r.start as usize..r.end as usize].to_vec()),
+            )
+            .unwrap_err();
+        assert!(e.contains("maxFetch"), "{e}");
+        assert!(s.stats().bytes_fetched <= 8 * 1024, "{:?}", s.stats());
+        // Within the budget it completes, and the budget is per operation.
+        for _ in 0..3 {
+            s.run_blocking(
+                || owned(s.read_at(10 * 1024, 3000)),
+                |r| Ok(data[r.start as usize..r.end as usize].to_vec()),
+            )
+            .unwrap()
+            .unwrap();
+        }
+    }
+
+    #[test]
+    fn a_failed_fetch_is_an_error_not_data() {
+        let data = file(4096);
+        let s = LazyStorage::new(data.len() as u64, config(1024, 1 << 20));
+        let e = s
+            .run_blocking(|| owned(s.read_at(0, 8)), |_| Err("HTTP 500".into()))
+            .unwrap_err();
+        assert_eq!(e, "HTTP 500");
+        let e = s
+            .run_blocking(|| owned(s.read_at(0, 8)), |_| Ok(vec![0; 10]))
+            .unwrap_err();
+        assert!(e.contains("asked for 1024 bytes"), "{e}");
+        assert_eq!(s.stats().cached_bytes, 0);
+        drop(data);
+    }
+}
diff --git a/crates/clawhdf5-wasm/src/lib.rs b/crates/clawhdf5-wasm/src/lib.rs
index 7c6b4a9..6a512a2 100644
--- a/crates/clawhdf5-wasm/src/lib.rs
+++ b/crates/clawhdf5-wasm/src/lib.rs
@@ -1,7 +1,7 @@
 //! clawhdf5's HDF5 reader for JavaScript, via `wasm-bindgen`.
 //!
 //! ```js
-//! import init, { open } from "./pkg/clawhdf5_wasm.js";
+//! import init, { open, openUrl } from "./pkg/clawhdf5_wasm.js";
 //! await init();
 //! const file = open(new Uint8Array(await blob.arrayBuffer()));
 //! file.list("/");            // [{ name, kind: "group" | "dataset" }]
@@ -10,6 +10,12 @@
 //! file.read("/x");           // { shape, dtype, data: Float64Array | ... | string[] }
 //! file.readHyperslab("/x", [0, 0], [10, 10]);    // stride, block optional
 //! file.free();
+//!
+//! // A file on a web server, read by HTTP range requests as needed: the
+//! // same methods, returning promises.
+//! const remote = await openUrl("https://example.org/data.h5");
+//! await remote.list("/");
+//! remote.stats();            // { requests, bytesFetched, size, ... }
 //! ```
 //!
 //! Numeric data comes back in the typed array of the stored width
@@ -18,15 +24,23 @@
 //! Anything else is a thrown `Error` naming the datatype. Only the reader is
 //! exposed: nothing here writes files.
 //!
-//! The logic lives in [`core`], which is plain Rust and tested natively.
+//! The logic lives in [`core`] and [`lazy`], which are plain Rust and tested
+//! natively; `js/remote.js` does the HTTP.
 
 pub mod core;
+pub mod lazy;
+
+use std::ops::Range;
+use std::rc::Rc;
+use std::sync::Arc;
 
 use clawhdf5::AttrValue;
-use js_sys::{Array, Object, Reflect};
+use js_sys::{Array, Object, Promise, Reflect, Uint8Array};
 use wasm_bindgen::prelude::*;
+use wasm_bindgen_futures::future_to_promise;
 
-use crate::core::{Data, Hyperslab, Reader};
+use crate::core::{Attr, Child, Data, DatasetInfo, Hyperslab, Reader};
+use crate::lazy::{LazyConfig, LazyStorage, Step};
 
 /// JavaScript numbers are exact up to 2^53.
 const MAX_SAFE_INTEGER: f64 = 9_007_199_254_740_991.0;
@@ -35,11 +49,30 @@ fn js_err(msg: String) -> JsError {
     JsError::new(&msg)
 }
 
+/// A JavaScript exception (from `fetch`, or `js/remote.js`) as a `JsError`
+/// with its message.
+fn js_exception(e: JsValue) -> JsError {
+    let msg = e
+        .dyn_ref::<js_sys::Error>()
+        .map(|e| String::from(e.message()))
+        .or_else(|| e.as_string())
+        .unwrap_or_else(|| format!("{e:?}"));
+    JsError::new(&msg)
+}
+
 fn set(obj: &Object, key: &str, value: impl Into<JsValue>) {
     // Defining a property on a fresh plain object cannot fail.
     Reflect::set(obj, &JsValue::from_str(key), &value.into()).unwrap_throw();
 }
 
+fn get(obj: &JsValue, key: &str) -> JsValue {
+    if obj.is_object() {
+        Reflect::get(obj, &JsValue::from_str(key)).unwrap_or(JsValue::UNDEFINED)
+    } else {
+        JsValue::UNDEFINED
+    }
+}
+
 fn shape_to_js(shape: &[u64]) -> Array {
     shape.iter().map(|&d| JsValue::from_f64(d as f64)).collect()
 }
@@ -58,6 +91,20 @@ fn indices_from_js(what: &str, v: &[f64]) -> Result<Vec<u64>, JsError> {
         .collect()
 }
 
+fn slab_from_js(
+    start: &[f64],
+    count: &[f64],
+    stride: Option<Vec<f64>>,
+    block: Option<Vec<f64>>,
+) -> Result<Hyperslab, JsError> {
+    Ok(Hyperslab {
+        start: indices_from_js("start", start)?,
+        count: indices_from_js("count", count)?,
+        stride: stride.map(|s| indices_from_js("stride", &s)).transpose()?,
+        block: block.map(|b| indices_from_js("block", &b)).transpose()?,
+    })
+}
+
 fn data_to_js(data: Data) -> JsValue {
     match data {
         Data::F32(v) => js_sys::Float32Array::from(&v[..]).into(),
@@ -110,6 +157,72 @@ fn attr_to_js(value: AttrValue) -> (JsValue, Option<String>) {
     }
 }
 
+fn list_to_js(children: Vec<Child>) -> Array {
+    children
+        .into_iter()
+        .map(|c| {
+            let o = Object::new();
+            set(&o, "name", c.name);
+            set(&o, "kind", c.kind.as_str());
+            JsValue::from(o)
+        })
+        .collect()
+}
+
+fn info_to_js(i: DatasetInfo) -> Object {
+    let o = Object::new();
+    set(&o, "shape", shape_to_js(&i.shape));
+    let max: JsValue = match i.maxshape {
+        None => JsValue::NULL,
+        Some(dims) => dims
+            .into_iter()
+            .map(|d| d.map_or(JsValue::NULL, |d| JsValue::from_f64(d as f64)))
+            .collect::<Array>()
+            .into(),
+    };
+    set(&o, "maxshape", max);
+    set(&o, "dtype", i.dtype);
+    set(&o, "elementShape", shape_to_js(&i.element_shape));
+    o
+}
+
+fn attrs_to_js(attrs: Vec<Attr>) -> Array {
+    attrs
+        .into_iter()
+        .map(|a| {
+            let o = Object::new();
+            set(&o, "name", a.name);
+            let (value, dtype) = attr_to_js(a.value);
+            set(&o, "value", value);
+            set(&o, "dtype", dtype.map_or(JsValue::NULL, JsValue::from));
+            JsValue::from(o)
+        })
+        .collect()
+}
+
+fn errors_to_js(errors: Vec<String>) -> Array {
+    errors.into_iter().map(JsValue::from).collect()
+}
+
+/// A read's values with the dataset's datatype.
+fn values_to_js((dtype, a): (String, core::Array)) -> Object {
+    let o = Object::new();
+    set(&o, "shape", shape_to_js(&a.shape));
+    set(&o, "dtype", dtype);
+    set(&o, "data", data_to_js(a.data));
+    o
+}
+
+/// Read the dataset at `path` (whole, or `slab`) with its datatype.
+fn read_values(
+    r: &Reader,
+    path: &str,
+    slab: Option<&Hyperslab>,
+) -> core::Result<(String, core::Array)> {
+    let dtype = r.info(path)?.dtype;
+    Ok((dtype, r.read(path, slab)?))
+}
+
 /// An open HDF5 (or NetCDF-4) file.
 #[wasm_bindgen]
 pub struct H5File {
@@ -145,38 +258,13 @@ impl H5File {
 
     /// The group's members: `[{ name, kind }]`, groups first.
     pub fn list(&self, path: &str) -> Result<Array, JsError> {
-        Ok(self
-            .inner
-            .list(path)
-            .map_err(js_err)?
-            .into_iter()
-            .map(|c| {
-                let o = Object::new();
-                set(&o, "name", c.name);
-                set(&o, "kind", c.kind.as_str());
-                JsValue::from(o)
-            })
-            .collect())
+        Ok(list_to_js(self.inner.list(path).map_err(js_err)?))
     }
 
     /// `{ shape, maxshape, dtype, elementShape }`. `maxshape` is `null`
     /// when not recorded, with `null` for each unlimited dimension.
     pub fn info(&self, path: &str) -> Result<Object, JsError> {
-        let i = self.inner.info(path).map_err(js_err)?;
-        let o = Object::new();
-        set(&o, "shape", shape_to_js(&i.shape));
-        let max: JsValue = match i.maxshape {
-            None => JsValue::NULL,
-            Some(dims) => dims
-                .into_iter()
-                .map(|d| d.map_or(JsValue::NULL, |d| JsValue::from_f64(d as f64)))
-                .collect::<Array>()
-                .into(),
-        };
-        set(&o, "maxshape", max);
-        set(&o, "dtype", i.dtype);
-        set(&o, "elementShape", shape_to_js(&i.element_shape));
-        Ok(o)
+        Ok(info_to_js(self.inner.info(path).map_err(js_err)?))
     }
 
     /// `[{ name, value, dtype }]`, sorted by name. Scalars are `number`
@@ -185,31 +273,21 @@ impl H5File {
     /// and its `dtype`; one that could not be read at all is reported by
     /// [`attrErrors`](Self::attr_errors).
     pub fn attrs(&self, path: &str) -> Result<Array, JsError> {
-        let (attrs, _) = self.inner.attrs(path).map_err(js_err)?;
-        Ok(attrs
-            .into_iter()
-            .map(|a| {
-                let o = Object::new();
-                set(&o, "name", a.name);
-                let (value, dtype) = attr_to_js(a.value);
-                set(&o, "value", value);
-                set(&o, "dtype", dtype.map_or(JsValue::NULL, JsValue::from));
-                JsValue::from(o)
-            })
-            .collect())
+        Ok(attrs_to_js(self.inner.attrs(path).map_err(js_err)?.0))
     }
 
     /// Messages for attributes that could not be read.
     #[wasm_bindgen(js_name = attrErrors)]
     pub fn attr_errors(&self, path: &str) -> Result<Array, JsError> {
-        let (_, errors) = self.inner.attrs(path).map_err(js_err)?;
-        Ok(errors.into_iter().map(JsValue::from).collect())
+        Ok(errors_to_js(self.inner.attrs(path).map_err(js_err)?.1))
     }
 
     /// The whole dataset: `{ shape, dtype, data }`, `data` in row-major
     /// order.
     pub fn read(&self, path: &str) -> Result<Object, JsError> {
-        self.read_impl(path, None)
+        Ok(values_to_js(
+            read_values(&self.inner, path, None).map_err(js_err)?,
+        ))
     }
 
     /// A regular hyperslab (`H5Sselect_hyperslab`): `stride` and `block`
@@ -223,22 +301,358 @@ impl H5File {
         stride: Option<Vec<f64>>,
         block: Option<Vec<f64>>,
     ) -> Result<Object, JsError> {
-        let slab = Hyperslab {
-            start: indices_from_js("start", &start)?,
-            count: indices_from_js("count", &count)?,
-            stride: stride.map(|s| indices_from_js("stride", &s)).transpose()?,
-            block: block.map(|b| indices_from_js("block", &b)).transpose()?,
-        };
-        self.read_impl(path, Some(&slab))
-    }
-
-    fn read_impl(&self, path: &str, slab: Option<&Hyperslab>) -> Result<Object, JsError> {
-        let dtype = self.inner.info(path).map_err(js_err)?.dtype;
-        let a = self.inner.read(path, slab).map_err(js_err)?;
+        let slab = slab_from_js(&start, &count, stride, block)?;
+        Ok(values_to_js(
+            read_values(&self.inner, path, Some(&slab)).map_err(js_err)?,
+        ))
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Remote files: openUrl.
+
+#[wasm_bindgen(module = "/js/remote.js")]
+extern "C" {
+    #[wasm_bindgen(catch)]
+    async fn probe(url: &str, first_len: f64, opts: &JsValue) -> Result<JsValue, JsValue>;
+
+    #[wasm_bindgen(catch, js_name = fetchRanges)]
+    async fn fetch_ranges(
+        url: &str,
+        ranges: Vec<f64>,
+        opts: &JsValue,
+        validator: &JsValue,
+        length: f64,
+    ) -> Result<JsValue, JsValue>;
+}
+
+/// Where a remote file's bytes come from.
+struct Http {
+    url: String,
+    opts: JsValue,
+    /// ETag or Last-Modified at open (`null` if the server sent neither).
+    validator: JsValue,
+    length: u64,
+    /// Requests the probe made (1, or 2 with a HEAD for the length).
+    probe_requests: u64,
+}
+
+enum Source {
+    /// Read by range requests through a restartable cache.
+    Lazy {
+        http: Http,
+        storage: Arc<LazyStorage>,
+        reader: Reader,
+    },
+    /// The server ignored `Range`: the whole file, downloaded at open.
+    Whole {
+        reader: Reader,
+        size: u64,
+        requests: u64,
+    },
+}
+
+impl Http {
+    /// Fetch `ranges` and hand them to `storage`.
+    async fn fetch(&self, storage: &LazyStorage, ranges: &[Range<u64>]) -> Result<(), JsError> {
+        let flat: Vec<f64> = ranges
+            .iter()
+            .flat_map(|r| [r.start as f64, r.end as f64])
+            .collect();
+        let got = fetch_ranges(
+            &self.url,
+            flat,
+            &self.opts,
+            &self.validator,
+            self.length as f64,
+        )
+        .await
+        .map_err(js_exception)?;
+        let got = Array::from(&got);
+        if got.length() as usize != ranges.len() {
+            return Err(js_err(format!(
+                "fetchRanges returned {} ranges for {}",
+                got.length(),
+                ranges.len()
+            )));
+        }
+        for (r, bytes) in ranges.iter().zip(got.iter()) {
+            let bytes = Uint8Array::new(&bytes).to_vec();
+            storage.supply_range(r, &bytes).map_err(js_err)?;
+        }
+        Ok(())
+    }
+
+    /// Run `f` over `storage` until it has every byte it reads.
+    async fn drive<T>(
+        &self,
+        storage: &LazyStorage,
+        mut f: impl FnMut() -> T,
+    ) -> Result<T, JsError> {
+        let op = storage.operation();
+        loop {
+            match storage.attempt(&mut f) {
+                Step::Done(v) => return Ok(v),
+                Step::Need(ranges) => {
+                    op.charge(&ranges).map_err(js_err)?;
+                    self.fetch(storage, &ranges).await?
+                }
+            }
+        }
+    }
+}
+
+impl Source {
+    async fn run<T>(&self, op: impl Fn(&Reader) -> core::Result<T>) -> Result<T, JsError> {
+        match self {
+            Source::Whole { reader, .. } => op(reader).map_err(js_err),
+            Source::Lazy {
+                http,
+                storage,
+                reader,
+            } => http.drive(storage, || op(reader)).await?.map_err(js_err),
+        }
+    }
+}
+
+/// A non-negative integer option, or `None` when not given.
+fn int_opt(opts: &JsValue, key: &str, min: f64, max: f64) -> Result<Option<u64>, JsError> {
+    let v = get(opts, key);
+    if v.is_undefined() || v.is_null() {
+        return Ok(None);
+    }
+    match v.as_f64() {
+        Some(x) if x.fract() == 0.0 && (min..=max).contains(&x) => Ok(Some(x as u64)),
+        _ => Err(js_err(format!(
+            "openUrl: {key} must be an integer from {min} to {max}"
+        ))),
+    }
+}
+
+/// Most bytes `maxFetch` and `maxDownload` may allow: 1 GiB. wasm32 has
+/// 4 GiB of memory and no buffer past 2 GiB, and what is fetched is held
+/// while it is decoded.
+const MAX_FETCH_LIMIT: u64 = 1 << 30;
+
+/// The largest file `openUrl` reads by ranges: on wasm32, 4 GiB - 1 bytes.
+/// The format code turns file offsets into `usize` to use them (with a
+/// clean error past it, see scripts/check-32bit-casts.sh), so on a 32-bit
+/// target nothing at 4 GiB or beyond can be read; a larger file is refused
+/// at open rather than failing on whichever read reaches past 4 GiB. On
+/// 64-bit targets it is 2^53 - 1, the largest offset a JavaScript number
+/// holds exactly.
+const MAX_REMOTE_LENGTH: u64 = if (usize::MAX as u64) < MAX_SAFE_INTEGER as u64 {
+    usize::MAX as u64
+} else {
+    MAX_SAFE_INTEGER as u64
+};
+
+fn config_from(opts: &JsValue) -> Result<LazyConfig, JsError> {
+    let mut c = LazyConfig::default();
+    if let Some(b) = int_opt(opts, "blockSize", 512.0, (64u64 << 20) as f64)? {
+        c.block_size = b;
+    }
+    if let Some(n) = int_opt(opts, "cacheSize", 0.0, MAX_SAFE_INTEGER)? {
+        c.capacity = n;
+    }
+    if let Some(n) = int_opt(opts, "maxFetch", 512.0, MAX_FETCH_LIMIT as f64)? {
+        c.max_fetch = n;
+    }
+    // Read by remote.js; checked here so a value wasm32 cannot hold is an
+    // option error rather than a download that cannot be kept.
+    int_opt(opts, "maxDownload", 0.0, MAX_FETCH_LIMIT as f64)?;
+    // Also read by remote.js (which checks it too, for direct callers).
+    int_opt(opts, "parallel", 1.0, 1024.0)?;
+    Ok(c)
+}
+
+/// Open the HDF5 file at `url` without downloading it: its bytes are
+/// fetched with HTTP `Range` requests as the methods of the returned
+/// [`RemoteFile`] need them, through a block cache.
+///
+/// `opts` (all optional):
+/// - `blockSize` — bytes per request block, 512 to 64 MiB (default 1 MiB);
+/// - `cacheSize` — bytes of blocks kept between calls (default 64 MiB);
+/// - `maxFetch` — most bytes one call may fetch, and so the longest single
+///   read, up to 1 GiB (default 512 MiB): a call that would fetch more
+///   fails before fetching it;
+/// - `fallback` — `"download"` (default) reads the whole file when the
+///   server ignores `Range` (answers 200), up to `maxDownload` bytes
+///   (default 512 MiB, at most 1 GiB); `"error"` refuses such a server;
+/// - `headers`, `credentials` — passed to every `fetch` (`headers` as
+///   `fetch` takes them: a `Headers`, `[name, value]` pairs or an object);
+/// - `parallel` — range requests in flight at once, 1 to 1024 (default
+///   6); when one fails the others are aborted;
+/// - `fetch` — a `fetch`-compatible function to use instead of the global.
+///
+/// Cross-origin servers must allow CORS and expose `Content-Range` (or
+/// answer `HEAD` with `Content-Length`). A file may be up to 4 GiB - 1
+/// bytes long (wasm32 offsets); a longer one is refused at open. A whole-dataset `read` that would use more than 1 GiB of
+/// memory ([`core::MAX_READ_BYTES`]) is refused: read it in parts with
+/// `readHyperslab`.
+#[wasm_bindgen(js_name = openUrl)]
+pub async fn open_url(url: String, opts: JsValue) -> Result<RemoteFile, JsError> {
+    let config = config_from(&opts)?;
+    let p = probe(&url, config.block_size as f64, &opts)
+        .await
+        .map_err(js_exception)?;
+    let requests = get(&p, "requests").as_f64().unwrap_or(1.0) as u64;
+    let whole = get(&p, "whole");
+    if !whole.is_undefined() {
+        let bytes = Uint8Array::new(&whole).to_vec();
+        let size = bytes.len() as u64;
+        let reader = Reader::open(bytes).map_err(js_err)?;
+        return Ok(RemoteFile {
+            inner: Rc::new(Source::Whole {
+                reader,
+                size,
+                requests,
+            }),
+        });
+    }
+    let length = get(&p, "length")
+        .as_f64()
+        .filter(|x| x.fract() == 0.0 && (0.0..=MAX_SAFE_INTEGER).contains(x))
+        .ok_or_else(|| js_err(format!("{url}: the server gave no usable file size")))?;
+    if length as u64 > MAX_REMOTE_LENGTH {
+        return Err(js_err(format!(
+            "{url} is {length} bytes; openUrl reads files of up to {MAX_REMOTE_LENGTH} bytes \
+             (4 GiB - 1: the WebAssembly reader addresses a file with 32-bit offsets)"
+        )));
+    }
+    let http = Http {
+        url,
+        opts,
+        validator: get(&p, "validator"),
+        length: length as u64,
+        probe_requests: requests,
+    };
+    let storage = Arc::new(LazyStorage::new(http.length, config));
+    let first = Uint8Array::new(&get(&p, "first")).to_vec();
+    storage.supply(0, &first).map_err(js_err)?;
+    let s = storage.clone();
+    let reader = http
+        .drive(&storage, || Reader::open_storage(s.clone()))
+        .await?
+        .map_err(js_err)?;
+    Ok(RemoteFile {
+        inner: Rc::new(Source::Lazy {
+            http,
+            storage,
+            reader,
+        }),
+    })
+}
+
+/// A file opened with [`openUrl`](open_url): the methods of [`H5File`],
+/// each returning a `Promise` (it may have to fetch bytes first).
+#[wasm_bindgen]
+pub struct RemoteFile {
+    inner: Rc<Source>,
+}
+
+impl RemoteFile {
+    /// Run `op` (fetching what it needs) and convert its result.
+    fn call<T: 'static>(
+        &self,
+        op: impl Fn(&Reader) -> core::Result<T> + 'static,
+        to_js: impl FnOnce(T) -> JsValue + 'static,
+    ) -> Promise {
+        let inner = self.inner.clone();
+        future_to_promise(async move {
+            let v = inner.run(op).await.map_err(JsValue::from)?;
+            Ok(to_js(v))
+        })
+    }
+}
+
+#[wasm_bindgen]
+impl RemoteFile {
+    /// `"group"` or `"dataset"`.
+    #[wasm_bindgen(unchecked_return_type = "Promise<string>")]
+    pub fn kind(&self, path: String) -> Promise {
+        self.call(move |r| r.kind(&path), |k| k.as_str().into())
+    }
+
+    /// The group's members: `[{ name, kind }]`, groups first.
+    #[wasm_bindgen(unchecked_return_type = "Promise<Array<any>>")]
+    pub fn list(&self, path: String) -> Promise {
+        self.call(move |r| r.list(&path), |c| list_to_js(c).into())
+    }
+
+    /// `{ shape, maxshape, dtype, elementShape }`, as [`H5File::info`].
+    #[wasm_bindgen(unchecked_return_type = "Promise<any>")]
+    pub fn info(&self, path: String) -> Promise {
+        self.call(move |r| r.info(&path), |i| info_to_js(i).into())
+    }
+
+    /// `[{ name, value, dtype }]`, as [`H5File::attrs`].
+    #[wasm_bindgen(unchecked_return_type = "Promise<Array<any>>")]
+    pub fn attrs(&self, path: String) -> Promise {
+        self.call(move |r| r.attrs(&path), |a| attrs_to_js(a.0).into())
+    }
+
+    /// Messages for attributes that could not be read.
+    #[wasm_bindgen(js_name = attrErrors, unchecked_return_type = "Promise<Array<string>>")]
+    pub fn attr_errors(&self, path: String) -> Promise {
+        self.call(move |r| r.attrs(&path), |a| errors_to_js(a.1).into())
+    }
+
+    /// The whole dataset: `{ shape, dtype, data }`, as [`H5File::read`].
+    #[wasm_bindgen(unchecked_return_type = "Promise<any>")]
+    pub fn read(&self, path: String) -> Promise {
+        self.call(
+            move |r| read_values(r, &path, None),
+            |v| values_to_js(v).into(),
+        )
+    }
+
+    /// A regular hyperslab, as [`H5File::read_hyperslab`]. Only the chunks
+    /// (or the contiguous runs) the selection touches are fetched.
+    #[wasm_bindgen(js_name = readHyperslab, unchecked_return_type = "Promise<any>")]
+    pub fn read_hyperslab(
+        &self,
+        path: String,
+        start: Vec<f64>,
+        count: Vec<f64>,
+        stride: Option<Vec<f64>>,
+        block: Option<Vec<f64>>,
+    ) -> Result<Promise, JsError> {
+        let slab = slab_from_js(&start, &count, stride, block)?;
+        Ok(self.call(
+            move |r| read_values(r, &path, Some(&slab)),
+            |v| values_to_js(v).into(),
+        ))
+    }
+
+    /// What reading this file has cost so far: `{ lazy, size, requests,
+    /// bytesFetched, cachedBytes, passes }`. `lazy` is false when the
+    /// server ignored `Range` and the file was downloaded whole.
+    pub fn stats(&self) -> Object {
         let o = Object::new();
-        set(&o, "shape", shape_to_js(&a.shape));
-        set(&o, "dtype", dtype);
-        set(&o, "data", data_to_js(a.data));
-        Ok(o)
+        match &*self.inner {
+            Source::Lazy { http, storage, .. } => {
+                let st = storage.stats();
+                set(&o, "lazy", true);
+                set(&o, "size", http.length as f64);
+                set(
+                    &o,
+                    "requests",
+                    (st.requests.saturating_sub(1) + http.probe_requests) as f64,
+                );
+                set(&o, "bytesFetched", st.bytes_fetched as f64);
+                set(&o, "cachedBytes", st.cached_bytes as f64);
+                set(&o, "passes", st.passes as f64);
+            }
+            Source::Whole { size, requests, .. } => {
+                set(&o, "lazy", false);
+                set(&o, "size", *size as f64);
+                set(&o, "requests", *requests as f64);
+                set(&o, "bytesFetched", *size as f64);
+                set(&o, "cachedBytes", *size as f64);
+                set(&o, "passes", 0.0);
+            }
+        }
+        o
     }
 }
diff --git a/crates/clawhdf5-wasm/tests/lazy.rs b/crates/clawhdf5-wasm/tests/lazy.rs
new file mode 100644
index 0000000..0d0b536
--- /dev/null
+++ b/crates/clawhdf5-wasm/tests/lazy.rs
@@ -0,0 +1,578 @@
+//! The restartable ("NeedBytes") reader against the in-memory one: every
+//! file must list, describe and read the same through a [`LazyStorage`]
+//! that starts empty and is fed only the ranges its passes ask for, as the
+//! browser's `openUrl` feeds it from HTTP range requests.
+//!
+//! - Files written here with `FileBuilder`, at several block sizes (512 B
+//!   blocks make almost every structure read a miss).
+//! - The h5py/netCDF4 fixture of `examples/wasm-viewer/test/make_fixture.py`
+//!   (skipped without h5py, unless `CLAWHDF5_REQUIRE_INTEROP=1`;
+//!   `CLAWHDF5_PYTHON` names the interpreter).
+//! - `CLAWHDF5_WASM_CORPUS=dir[:dir...]`: every HDF5 file under those
+//!   directories up to 64 MiB (e.g. `conformance/.cache/corpus`).
+//!
+//! Also the request budget: listing and reading one small dataset of a large
+//! file fetches a few blocks, not the file.
+
+use std::ops::Range;
+use std::path::{Path, PathBuf};
+use std::process::Command;
+use std::sync::Arc;
+
+use clawhdf5::{AttrValue, FileBuilder};
+use clawhdf5_format::storage::CountingStorage;
+use clawhdf5_wasm::core::{Hyperslab, Kind, Reader};
+use clawhdf5_wasm::lazy::{LazyConfig, LazyStorage};
+
+/// The API the JavaScript side calls, one operation at a time.
+trait Api {
+    fn call<T>(&self, op: impl Fn(&Reader) -> T) -> T;
+}
+
+struct Local(Reader);
+
+impl Api for Local {
+    fn call<T>(&self, op: impl Fn(&Reader) -> T) -> T {
+        op(&self.0)
+    }
+}
+
+/// A lazily read file and the "server" it fetches from.
+struct Lazy {
+    data: Arc<Vec<u8>>,
+    storage: Arc<LazyStorage>,
+    reader: Reader,
+}
+
+fn fetch(data: &[u8], r: Range<u64>) -> Result<Vec<u8>, String> {
+    Ok(data[r.start as usize..r.end as usize].to_vec())
+}
+
+impl Lazy {
+    /// Open as `openUrl` does: the first block comes with the probe that
+    /// learns the length, then the open is run until it has its bytes.
+    fn open(data: Vec<u8>, config: LazyConfig) -> Result<Lazy, String> {
+        let data = Arc::new(data);
+        let storage = Arc::new(LazyStorage::new(data.len() as u64, config));
+        let first = (storage.config().block_size as usize).min(data.len());
+        storage.supply(0, &data[..first])?;
+        let s = storage.clone();
+        let reader =
+            storage.run_blocking(|| Reader::open_storage(s.clone()), |r| fetch(&data, r))??;
+        Ok(Lazy {
+            data,
+            storage,
+            reader,
+        })
+    }
+}
+
+impl Api for Lazy {
+    fn call<T>(&self, op: impl Fn(&Reader) -> T) -> T {
+        self.storage
+            .run_blocking(|| op(&self.reader), |r| fetch(&self.data, r))
+            .expect("serving from memory cannot fail")
+    }
+}
+
+/// Everything the viewer can show of a file, as text: each object's kind,
+/// listing, attributes (and attribute errors), dataset info, whole value
+/// and a hyperslab — or the error each gives.
+fn transcript(api: &impl Api) -> Vec<String> {
+    let mut out = Vec::new();
+    let mut todo = vec![("/".to_string(), 0usize)];
+    while let Some((path, depth)) = todo.pop() {
+        if out.len() > 4000 {
+            out.push("... (truncated)".into());
+            break;
+        }
+        let kind = api.call(|r| r.kind(&path));
+        out.push(format!("{path}: {kind:?}"));
+        out.push(format!("{path} attrs: {:?}", api.call(|r| r.attrs(&path))));
+        match kind {
+            Ok(Kind::Group) => {
+                let list = api.call(|r| r.list(&path));
+                out.push(format!("{path} list: {list:?}"));
+                if let Ok(children) = list
+                    && depth < 12
+                {
+                    for c in children.into_iter().rev() {
+                        let child = if path == "/" {
+                            format!("/{}", c.name)
+                        } else {
+                            format!("{path}/{}", c.name)
+                        };
+                        todo.push((child, depth + 1));
+                    }
+                }
+            }
+            Ok(Kind::Dataset) => {
+                let info = api.call(|r| r.info(&path));
+                out.push(format!("{path} info: {info:?}"));
+                let Ok(info) = info else { continue };
+                let n = info
+                    .shape
+                    .iter()
+                    .chain(&info.element_shape)
+                    .try_fold(1u64, |a, &d| a.checked_mul(d));
+                if n.is_none_or(|n| n > 4_000_000) {
+                    out.push(format!("{path}: not read ({n:?} values)"));
+                    continue;
+                }
+                out.push(format!(
+                    "{path} read: {:?}",
+                    api.call(|r| r.read(&path, None))
+                ));
+                if !info.shape.is_empty() && info.shape.iter().all(|&d| d > 1) {
+                    let slab = Hyperslab {
+                        start: info.shape.iter().map(|_| 1).collect(),
+                        count: info.shape.iter().map(|&d| d / 2).collect(),
+                        stride: None,
+                        block: None,
+                    };
+                    let part = api.call(|r| r.read(&path, Some(&slab)));
+                    out.push(format!("{path} slab: {part:?}"));
+                }
+            }
+            Err(_) => {}
+        }
+    }
+    out
+}
+
+/// The lazy transcript of `data` at `block` bytes per block equals the
+/// transcript of the same file through a range storage that has every byte
+/// (`CountingStorage`: the facade's `Storage` path, the one the lazy reader
+/// takes), and agrees with the in-memory one: the same values, and an error
+/// wherever it has one (a malformed file can fail at a different check,
+/// with a different message, when read by ranges). Returns what the lazy
+/// reader fetched and its transcript.
+fn check_equal(name: &str, data: &[u8], block: u64) -> (u64, u64, Vec<String>) {
+    let ctx = format!("{name} (blocks of {block} B)");
+    let ranged = Reader::open_storage(Arc::new(CountingStorage::new(data.to_vec())));
+    let local = Reader::open(data.to_vec());
+    let lazy = Lazy::open(data.to_vec(), config(block));
+    let (ranged, local, lazy) = match (ranged, local, lazy) {
+        (Ok(r), Ok(l), Ok(z)) => (r, l, z),
+        (Err(r), Err(_), Err(z)) => {
+            assert_eq!(z, r, "{ctx}: open error");
+            return (0, 0, Vec::new());
+        }
+        (r, l, z) => panic!(
+            "{ctx}: opens differently: ranged {:?}, in memory {:?}, lazily {:?}",
+            r.err(),
+            l.err(),
+            z.err()
+        ),
+    };
+    let got = transcript(&lazy);
+    let want = transcript(&Local(ranged));
+    for (i, (w, g)) in want.iter().zip(&got).enumerate() {
+        assert_eq!(g, w, "{ctx}, line {i}");
+    }
+    assert_eq!(got.len(), want.len(), "{ctx}: transcript length");
+    let local = transcript(&Local(local));
+    for (i, (l, g)) in local.iter().zip(&got).enumerate() {
+        let both_errors = match (l.split_once("Err("), g.split_once("Err(")) {
+            (Some((a, _)), Some((b, _))) => a == b,
+            _ => false,
+        };
+        assert!(
+            l == g || both_errors,
+            "{ctx}, line {i}: in memory\n  {l}\nlazily\n  {g}"
+        );
+    }
+    assert_eq!(got.len(), local.len(), "{ctx}: transcript length");
+    let st = lazy.storage.stats();
+    (st.requests, st.bytes_fetched, got)
+}
+
+fn config(block: u64) -> LazyConfig {
+    LazyConfig {
+        block_size: block,
+        // A small budget, so eviction between operations is exercised.
+        capacity: 16 * block,
+        max_request: 8 * block,
+        ..LazyConfig::default()
+    }
+}
+
+fn builder_file() -> Vec<u8> {
+    let mut b = FileBuilder::new();
+    b.create_dataset("grid")
+        .with_f64_data(&(0..20_000).map(f64::from).collect::<Vec<_>>())
+        .with_shape(&[100, 200])
+        .with_chunks(&[10, 25])
+        .with_deflate(4);
+    b.create_dataset("contiguous")
+        .with_i32_data(&(0..50_000).collect::<Vec<_>>());
+    b.create_dataset("bytes").with_u8_data(&[1, 2, 250]);
+    let mut g = b.create_group("sensors");
+    for i in 0..40 {
+        g.create_dataset(&format!("t{i}"))
+            .with_f32_data(&[i as f32, 1.5, -2.25]);
+    }
+    g.set_attr("location", AttrValue::String("lab".into()));
+    b.add_group(g.finish());
+    b.set_attr("version", AttrValue::I64(3));
+    b.set_attr("scale", AttrValue::F64Array(vec![0.5, 2.0]));
+    b.finish().unwrap()
+}
+
+#[test]
+fn builder_files_read_the_same_at_every_block_size() {
+    let data = builder_file();
+    for block in [512, 4096, 1 << 20] {
+        let (requests, _, lines) = check_equal("builder", &data, block);
+        assert!(requests > 0);
+        // The transcript covers every object, values included.
+        assert!(lines.iter().any(|l| l.starts_with("/grid read: Ok")));
+        assert!(lines.iter().any(|l| l.starts_with("/grid slab: Ok")));
+        assert!(lines.iter().any(|l| l.starts_with("/sensors/t39 read: Ok")));
+    }
+}
+
+#[test]
+fn garbage_fails_to_open_as_in_memory() {
+    check_equal("zeros", &[0u8; 5000], 512);
+    check_equal("empty", &[], 512);
+    let mut cut = builder_file();
+    cut.truncate(cut.len() / 3);
+    check_equal("truncated", &cut, 512);
+}
+
+/// Listing a large file and reading one small dataset fetches a few blocks,
+/// not the file.
+#[test]
+fn a_small_read_of_a_large_file_fetches_a_few_blocks() {
+    let mut b = FileBuilder::new();
+    b.create_dataset("small").with_f64_data(&[1.0, 2.0, 3.0]);
+    // 48 MB of raw data, written after the small dataset's metadata.
+    b.create_dataset("big")
+        .with_f64_data(&(0..6_000_000).map(f64::from).collect::<Vec<_>>());
+    let mut g = b.create_group("group");
+    g.create_dataset("inner").with_i32_data(&[7, 8]);
+    b.add_group(g.finish());
+    let data = b.finish().unwrap();
+    let lazy = Lazy::open(data.clone(), LazyConfig::default()).unwrap();
+    let list = lazy.call(|r| r.list("/")).unwrap();
+    assert_eq!(list.len(), 3);
+    assert_eq!(
+        format!("{:?}", lazy.call(|r| r.read("/small", None)).unwrap().data),
+        "F64([1.0, 2.0, 3.0])"
+    );
+    assert_eq!(
+        format!(
+            "{:?}",
+            lazy.call(|r| r.read("/group/inner", None)).unwrap().data
+        ),
+        "I32([7, 8])"
+    );
+    // A window of the big dataset reads only its block(s).
+    let slab = Hyperslab {
+        start: vec![3_000_000],
+        count: vec![4],
+        stride: None,
+        block: None,
+    };
+    assert_eq!(
+        format!(
+            "{:?}",
+            lazy.call(|r| r.read("/big", Some(&slab))).unwrap().data
+        ),
+        "F64([3000000.0, 3000001.0, 3000002.0, 3000003.0])"
+    );
+    let st = lazy.storage.stats();
+    eprintln!("{} bytes: {st:?}", data.len());
+    assert!(st.requests <= 6, "{st:?}");
+    assert!(st.bytes_fetched <= 6 << 20, "{st:?}");
+    assert!(st.bytes_fetched * 8 < data.len() as u64, "{st:?}");
+}
+
+fn python() -> String {
+    std::env::var("CLAWHDF5_PYTHON").unwrap_or_else(|_| "python3".to_string())
+}
+
+fn python_available() -> bool {
+    Command::new(python())
+        .args(["-c", "import h5py, netCDF4, numpy"])
+        .output()
+        .is_ok_and(|o| o.status.success())
+}
+
+#[test]
+fn h5py_and_netcdf4_files_read_the_same_lazily() {
+    if !python_available() {
+        assert!(
+            !std::env::var("CLAWHDF5_REQUIRE_INTEROP").is_ok_and(|v| v == "1"),
+            "CLAWHDF5_REQUIRE_INTEROP=1 but {} lacks h5py/netCDF4/numpy",
+            python()
+        );
+        eprintln!("skipping: {} lacks h5py/netCDF4/numpy", python());
+        return;
+    }
+    let dir = fixture_dir();
+    for name in ["fixture.h5", "fixture.nc"] {
+        let data = std::fs::read(dir.path().join(name)).unwrap();
+        for block in [512, 64 * 1024] {
+            let (_, _, lines) = check_equal(name, &data, block);
+            assert!(lines.iter().filter(|l| l.contains(" read: Ok")).count() >= 2);
+        }
+    }
+}
+
+/// The bytes of `data` at `r`, zero past its end: a server that claims
+/// the file is longer than it is.
+fn fetch_padded(data: &[u8], r: Range<u64>) -> Result<Vec<u8>, String> {
+    let mut out = vec![0u8; (r.end - r.start) as usize];
+    let len = data.len() as u64;
+    if r.start < len {
+        let end = r.end.min(len);
+        out[..(end - r.start) as usize].copy_from_slice(&data[r.start as usize..end as usize]);
+    }
+    Ok(out)
+}
+
+/// Sizes a hostile server or a large dataset can name are errors, never
+/// allocations that abort the wasm module: make_fixture.py's limits.h5 and
+/// hostile_vl.h5 (see write_limits there).
+#[test]
+fn size_limits_are_errors_not_aborts() {
+    if !python_available() {
+        assert!(
+            !std::env::var("CLAWHDF5_REQUIRE_INTEROP").is_ok_and(|v| v == "1"),
+            "CLAWHDF5_REQUIRE_INTEROP=1 but {} lacks h5py/netCDF4/numpy",
+            python()
+        );
+        eprintln!("skipping: {} lacks h5py/netCDF4/numpy", python());
+        return;
+    }
+    let dir = fixture_dir();
+
+    // Read whole, /huge_u8 would widen 2^28 values to 64 bits (2 GiB): an
+    // error naming readHyperslab, before its chunks are read. A window of
+    // it reads.
+    let data = std::fs::read(dir.path().join("limits.h5")).unwrap();
+    let n = (1u64 << 28) + 1024;
+    let window = Hyperslab {
+        start: vec![n - 4],
+        count: vec![4],
+        stride: None,
+        block: None,
+    };
+    let local = Reader::open(data.clone()).unwrap();
+    let lazy = Lazy::open(data, LazyConfig::default()).unwrap();
+    let before = lazy.storage.stats().requests;
+    for e in [
+        local.read("/huge_u8", None).unwrap_err(),
+        lazy.call(|r| r.read("/huge_u8", None)).unwrap_err(),
+    ] {
+        assert!(e.contains("readHyperslab"), "{e}");
+    }
+    assert_eq!(lazy.storage.stats().requests, before, "nothing fetched");
+    for part in [
+        local.read("/huge_u8", Some(&window)).unwrap(),
+        lazy.call(|r| r.read("/huge_u8", Some(&window))).unwrap(),
+    ] {
+        assert_eq!(format!("{:?}", part.data), "U8([0, 0, 0, 7])");
+    }
+
+    // A server that claims 3 GiB and a heap collection of 2 GiB + 4 KiB:
+    // reading the strings fails at once, fetching a few blocks.
+    let data = std::fs::read(dir.path().join("hostile_vl.h5")).unwrap();
+    let storage = Arc::new(LazyStorage::new(3 << 30, LazyConfig::default()));
+    let s = storage.clone();
+    let reader = storage
+        .run_blocking(
+            || Reader::open_storage(s.clone()),
+            |r| fetch_padded(&data, r),
+        )
+        .unwrap()
+        .unwrap();
+    let e = storage
+        .run_blocking(|| reader.read("/a", None), |r| fetch_padded(&data, r))
+        .unwrap()
+        .unwrap_err();
+    assert!(e.contains("maxFetch"), "{e}");
+    let st = storage.stats();
+    assert!(st.requests <= 4 && st.bytes_fetched <= 4 << 20, "{st:?}");
+}
+
+/// make_fixture.py's files, written to a temporary directory.
+fn fixture_dir() -> tempfile::TempDir {
+    let dir = tempfile::tempdir().unwrap();
+    let generator = Path::new(env!("CARGO_MANIFEST_DIR"))
+        .join("../../examples/wasm-viewer/test/make_fixture.py");
+    let out = Command::new(python())
+        .arg(&generator)
+        .arg(dir.path())
+        .output()
+        .unwrap();
+    assert!(
+        out.status.success(),
+        "{}",
+        String::from_utf8_lossy(&out.stderr)
+    );
+    dir
+}
+
+fn hdf5_files(dir: &Path, out: &mut Vec<PathBuf>) {
+    let Ok(entries) = std::fs::read_dir(dir) else {
+        return;
+    };
+    for e in entries.flatten() {
+        let p = e.path();
+        if p.is_dir() {
+            hdf5_files(&p, out);
+        } else if std::fs::read(&p)
+            .ok()
+            .is_some_and(|b| b.len() <= 64 << 20 && is_hdf5(&b))
+        {
+            out.push(p);
+        }
+    }
+}
+
+/// The HDF5 signature at 0 or a power-of-two user-block offset.
+fn is_hdf5(b: &[u8]) -> bool {
+    const SIG: &[u8] = b"\x89HDF\r\n\x1a\n";
+    let mut at = 0usize;
+    loop {
+        if b.get(at..at + 8) == Some(SIG) {
+            return true;
+        }
+        at = if at == 0 { 512 } else { at * 2 };
+        if at >= b.len() {
+            return false;
+        }
+    }
+}
+
+#[test]
+fn corpus_files_read_the_same_lazily() {
+    let Ok(dirs) = std::env::var("CLAWHDF5_WASM_CORPUS") else {
+        eprintln!("CLAWHDF5_WASM_CORPUS not set; skipping the corpus");
+        return;
+    };
+    let mut files = Vec::new();
+    for d in std::env::split_paths(&dirs) {
+        hdf5_files(&d, &mut files);
+    }
+    files.sort();
+    assert!(!files.is_empty(), "no HDF5 files under {dirs}");
+    let (mut requests, mut bytes, mut total) = (0u64, 0u64, 0u64);
+    for f in &files {
+        let data = std::fs::read(f).unwrap();
+        total += data.len() as u64;
+        let (r, b, _) = check_equal(&f.display().to_string(), &data, 64 * 1024);
+        requests += r;
+        bytes += b;
+    }
+    eprintln!(
+        "{} files ({total} bytes): {requests} requests, {bytes} bytes fetched",
+        files.len()
+    );
+}
+
+/// Passes and requests `list(path)` takes on a file opened lazily at
+/// `block`-byte blocks (the open not counted), checking the listing against
+/// the in-memory one.
+fn listing_cost(data: &[u8], path: &str, block: u64) -> (u64, u64) {
+    let want = Reader::open(data.to_vec()).unwrap().list(path).unwrap();
+    let lazy = Lazy::open(
+        data.to_vec(),
+        LazyConfig {
+            block_size: block,
+            ..LazyConfig::default()
+        },
+    )
+    .unwrap();
+    let before = lazy.storage.stats();
+    assert_eq!(lazy.call(|r| r.list(path)).unwrap(), want);
+    let after = lazy.storage.stats();
+    (
+        after.passes - before.passes,
+        after.requests - before.requests,
+    )
+}
+
+/// Listing a group reads every child's object header, and its index (B-tree
+/// and symbol table nodes, or B-tree v2 and heap blocks) before that. Each
+/// pass asks for every node of a level it is missing, not the first one
+/// only, so the passes (network round trips) grow with the depth of the
+/// index, not with the number of children: 2000 children with headers
+/// scattered over 512-byte blocks list in a handful of passes, where each
+/// header block used to cost its own.
+#[test]
+fn listing_a_large_group_takes_a_few_passes() {
+    let mut b = FileBuilder::new();
+    let mut g = b.create_group("many");
+    for i in 0..600 {
+        g.create_dataset(&format!("d{i}")).with_i32_data(&[i; 64]);
+    }
+    b.add_group(g.finish());
+    let data = b.finish().unwrap();
+    let (passes, requests) = listing_cost(&data, "/many", 512);
+    eprintln!("FileBuilder, 600 children: {passes} passes, {requests} requests");
+    assert!(passes <= 6, "{passes} passes");
+
+    if !python_available() {
+        return;
+    }
+    let dir = tempfile::tempdir().unwrap();
+    for libver in ["earliest", "latest"] {
+        let path = dir.path().join(format!("{libver}.h5"));
+        let script = format!(
+            "import h5py, numpy as np\n\
+             with h5py.File({:?}, 'w', libver='{libver}') as f:\n\
+             \x20   for i in range(2000):\n\
+             \x20       f.create_dataset('d%d' % i, data=np.full(256, i, np.float32))\n",
+            path.display().to_string()
+        );
+        let out = Command::new(python())
+            .args(["-c", &script])
+            .output()
+            .unwrap();
+        assert!(
+            out.status.success(),
+            "{}",
+            String::from_utf8_lossy(&out.stderr)
+        );
+        let data = std::fs::read(&path).unwrap();
+        let (passes, requests) = listing_cost(&data, "/", 512);
+        eprintln!("h5py libver={libver}, 2000 children: {passes} passes, {requests} requests");
+        assert!(passes <= 12, "{libver}: {passes} passes");
+    }
+}
+
+/// `CLAWHDF5_WASM_LIST_FILE=file.h5`: what listing the root group of that
+/// file costs lazily, at 1 MiB and 64 KiB blocks (a measurement, printed).
+#[test]
+fn listing_cost_of_a_given_file() {
+    let Ok(path) = std::env::var("CLAWHDF5_WASM_LIST_FILE") else {
+        return;
+    };
+    let data = std::fs::read(&path).unwrap();
+    for block in [1 << 20, 64 << 10] {
+        let lazy = Lazy::open(
+            data.clone(),
+            LazyConfig {
+                block_size: block,
+                ..LazyConfig::default()
+            },
+        )
+        .unwrap();
+        let open = lazy.storage.stats();
+        let n = lazy.call(|r| r.list("/")).unwrap().len();
+        let st = lazy.storage.stats();
+        eprintln!(
+            "{path} ({} bytes), {block}-byte blocks: open {} requests / {} passes; list('/') of {n}: {} passes, {} requests, {} bytes",
+            data.len(),
+            open.requests,
+            open.passes,
+            st.passes - open.passes,
+            st.requests - open.requests,
+            st.bytes_fetched - open.bytes_fetched
+        );
+    }
+}
diff --git a/docs/design/range-reads.md b/docs/design/range-reads.md
index fbb363e..4ab9ef5 100644
--- a/docs/design/range-reads.md
+++ b/docs/design/range-reads.md
@@ -20,13 +20,20 @@ change. Progress: M1, first part (the `Storage` trait and the metadata
 parsers listed in `CHANGELOG.md` under "Range reads, milestone M1") is done;
 group B-tree v2 lookups, dense groups and the facade are not converted yet.
 Later the same day (branch `feat/p3-editor-coverage`) two reader fixes touched
+
+object stores) and URLs in `h5rs` (see the M3 status below). M4 is done on
+branch `feat/p3-m4-wasm-lazy` (2026-09-27): `openUrl` in the browser
+reader, through the restartable `NeedBytes` mode (see the M4 status
+below). M5 (SWMR) is not started.
+Also on 2026-09-26 (branch `feat/p3-editor-coverage`) two reader fixes touched
 converted code without changing the plan: object-header continuation chunks
 are followed without recursion (still one bounded `read_at` per chunk), and
 implicit chunk indexes are addressed over the maximum chunk grid (in
 `chunked_read`, an M2 module). The in-place editor (`FileEditor`) keeps
-working on the whole file in memory; it is not part of this design. Every count below was
-taken on `tank` on 2026-09-26 at commit `de2a53f`, with the commands given
-next to it. No timing numbers appear here on purpose: the machine was shared
+working on the whole file in memory; it is not part of this design. Every
+count in §1–§2 was taken on `tank` on 2026-09-26 at commit `de2a53f`, and
+every count in a milestone's status on the date it gives, with the
+commands given next to it. No timing numbers appear here on purpose: the machine was shared
 with other build jobs when this was written.
 
 ## The problem
@@ -536,6 +543,70 @@ fast path within benchmark noise.
   Worker, no synchronous XHR — the thing h5wasm's lazy files need). Falls back
   to a whole download when the server does not answer 206.
 - `examples/wasm-viewer`: open by URL.
+- *Status 2026-09-27:* done on branch `feat/p3-m4-wasm-lazy`, as planned,
+  with these choices:
+  - **NeedBytes, not a Worker.** `clawhdf5_wasm::lazy::LazyStorage` is a
+    `Storage` over the blocks fetched so far. A call (open, list, read)
+    runs as a pass; a read that misses records its blocks and fails with
+    a storage error. The error itself is not the signal: parsers catch
+    errors and carry on (a listing leaves out a link it cannot resolve),
+    so *any* pass that recorded a miss is thrown away, whatever it
+    returned, and re-run once the missing ranges have been fetched
+    (`attempt` → `Step::Need(ranges)` → `supply` → again). A Worker with
+    synchronous XHR would have kept the parsers' single pass, but it
+    needs the page to run the reader off the main thread (a second
+    module, messages for every call and every typed array copied back),
+    since synchronous XHR can return binary data only there; the
+    restartable loop costs a
+    re-parse per wave of misses instead, which is CPU, not network.
+  - **Progress:** no block is evicted while a call is in flight
+    (`LazyStorage::operation`), so each pass that does not finish asks for
+    at least one new block, and a call ends after at most one pass per
+    block it reads. The budget (`cacheSize`, 64 MiB) is applied between
+    calls, raw-data blocks (`read_ranges`, or reads longer than a block)
+    evicted before metadata. The price: a call holds everything it reads
+    until it finishes.
+  - **Not `clawhdf5-remote`'s `BlockCache`.** It fetches through its
+    backend by blocking, evicts during a read, and does not keep reads
+    that miss more than half its budget; a restartable pass needs every
+    block it has read to be there when it is re-run. The block
+    arithmetic and coalescing (runs of consecutive blocks, a one-block
+    hole filled, at most 8 MiB per request) follow it; the cache is
+    about 450 lines (with its documentation) in the wasm crate, with no
+    in-flight tracking or HTTP. (A hole already cached is not filled:
+    it would be fetched again.)
+  - **The HTTP is JavaScript** (`crates/clawhdf5-wasm/js/remote.js`,
+    shipped as a wasm-bindgen snippet): `fetch` with `Range`, six
+    requests at a time, every answer checked (206, `Content-Range` when
+    visible, body length, the ETag/Last-Modified and length of the
+    first answer). A `200` to the first request is kept as the whole
+    file up to `maxDownload` (512 MiB), or refused with
+    `fallback: "error"`. The first block comes with the request that
+    learns the length, as in M3.
+  - Tested (tank, 2026-09-27): natively, `cargo test -p clawhdf5-wasm
+    --test lazy` compares what the viewer shows of each file (kinds,
+    listings, attributes, info, whole reads, hyperslabs) read lazily at
+    512 B to 1 MiB blocks with the in-memory reader and the facade's
+    range-storage path, for built files, the h5py/netCDF4 fixture and,
+    with `CLAWHDF5_WASM_CORPUS=conformance/.cache/corpus`, 656 corpus
+    files. The built package, under Node and headless Chromium
+    (`WASM_BIG_MB=200 bash examples/wasm-viewer/test/run.sh`, against
+    `test/serve.py`, which counts requests): every fixture check over
+    HTTP; in a 200 MB h5py file, listing, two small reads, a group's
+    attributes and a 10-value window of the 25-million-value dataset
+    took 5 requests, 6 MiB; with the corpus variable, 622 files agree
+    with `open(bytes)`.
+  - After review (2026-09-27): a call may fetch at most `maxFetch`
+    (512 MiB, at most 1 GiB) and a longer single read is refused before
+    fetching; `read()` refuses a decode over 1 GiB; a file of 4 GiB or
+    more is refused at open on wasm32 (offsets become `usize`); every
+    response body is cut off at the length asked for. Parsers that walk
+    siblings (B-tree v1/v2 collectors, symbol table nodes, dense links,
+    the wasm listing's child headers) read the siblings after the first
+    failure before returning it, so a pass asks for a whole level's
+    missing blocks: listing 3000 datasets went from 185 passes to 6.
+    The 32-bit risk below is covered by a Node test that reads data at
+    3 GiB from a mock server and is refused a 4 GiB file.
 
 **M5 — SWMR and growth (later, separate design).** `Storage::len()` may grow;
 add `File::refresh()` that re-reads the superblock/EOF and invalidates cached
diff --git a/docs/known-issues.md b/docs/known-issues.md
index d4136d7..9eeb4fc 100644
--- a/docs/known-issues.md
+++ b/docs/known-issues.md
@@ -948,6 +948,11 @@ cache, but:
   `LazyFile`, `MmapFile` and the wasm bindings still read a whole file
   (`h5rs` and the Python bindings read through `File::storage`, and take
   URLs: `h5rs` with its `remote` feature, Python with `clawhdf5.File(url)`).
+
+  `LazyFile`, `MmapFile` and the Python bindings still read a whole file
+  (`h5rs` reads through `File::storage`, and takes URLs with its `remote`
+  feature; the wasm reader's `openUrl` reads by range requests since
+  2026-09-27, its `open(bytes)` takes a whole file).
 - The file's length is read once, at open: a growing file (SWMR) is not
   followed (milestone M5). A remote file is pinned at open, so one that
   grows is `RemoteError::FileChanged`.
@@ -969,6 +974,10 @@ cache, but:
   built with `--features https` (rustls with ring, which compiles C), and
   `s3://`, `gs://`, `az://` the `s3`, `gcs`, `azure` features (aws-lc-rs).
   The Python tests run against an in-process `http.server` only.
+
+- **Python cannot open URLs yet.** `clawhdf5.File` (PyO3) parses through
+  `File::as_bytes`, which a remote file does not have. (The browser can
+  since 2026-09-27: `clawhdf5-wasm`'s `openUrl`, below.)
 - **The block size is fixed** (1 MiB unless `CacheConfig` says otherwise).
   The design's policy of using a paged file's page size as the block size
   is not implemented, and only the first block is read ahead.
@@ -1007,10 +1016,66 @@ cache, but:
 
 ## `clawhdf5-wasm` (browser) limits
 
-**Status:** open (by design for now; added 2026-09-26).
+**Status:** open (by design for now; added 2026-09-26, `openUrl` 2026-09-27).
 
-- The whole file is held in memory: `open()` takes its bytes. There are no
-  HTTP range reads, so a multi-GB file does not fit a browser tab.
+- `open()` holds the whole file in memory (it takes its bytes), so a
+  multi-GB local file does not fit a browser tab. A file on a web server
+  can be opened with `openUrl()` instead, which fetches only the byte
+  ranges each call needs (milestone M4 of `docs/design/range-reads.md`),
+  with these limits:
+  - **Round trips:** a call runs as passes over the blocks fetched so far
+    and is re-run after each wave of misses, so a call costs one round
+    trip per wave, not one for everything: a chunk index is walked a
+    level (or a node) per round trip, while the chunks of a read are
+    fetched together. Listing a group asks for every child's object
+    header, and every node of a level of the group's index, in one pass
+    (since 2026-09-27; it was one round trip per header block): 3000
+    datasets of an h5py file took 6 passes at 1 MiB blocks, 9 for a
+    `libver="latest"` file (dense links). Each pass re-parses what the
+    call reads (CPU, not network). With headers spread through the file
+    (h5py writes each next to its data) a listing still fetches most of
+    the file at 1 MiB blocks; a smaller `blockSize` fetches less.
+  - **Memory:** a call keeps every block it reads until it finishes (the
+    cache budget applies between calls). It may fetch at most `maxFetch`
+    bytes (512 MiB by default, at most 1 GiB), and a single read longer
+    than that fails before anything is fetched: a hostile server cannot
+    make the page fetch or allocate what a file's lengths claim. `read()`
+    of a dataset that would take more than 1 GiB while it is decoded
+    (stored bytes, the values widened to 64 bits, the result) fails,
+    naming `readHyperslab`; read windows of large datasets. On wasm32 a
+    buffer past 2 GiB cannot exist and a failed allocation aborts the
+    whole module (every open file on the page), which these limits keep
+    from happening; before 2026-09-27 both did abort it.
+  - **File size:** at most 4 GiB - 1 bytes; a larger file is refused at
+    open. The format code turns file offsets into `usize` to use them
+    (with a clean error past it), which is 32 bits on wasm32, so nothing
+    at 4 GiB or beyond could be read. Offsets between 2 and 4 GiB are
+    tested (with a mock server); files above 200 MB have not been served
+    for real.
+  - **Cross-origin servers** must allow CORS for the page's origin and
+    either expose `Content-Range` (`Access-Control-Expose-Headers`) or
+    answer `HEAD` with `Content-Length`. The file is pinned at open by its
+    `ETag` or `Last-Modified` (and its length): when the page cannot see
+    either header, only a change of length is detected.
+  - **A server without range support** (it answers `200`) costs a whole
+    download, up to `maxDownload` (512 MiB, at most 1 GiB), or an error
+    with `fallback: "error"`. Every body, this one and each `206`, is read
+    as it arrives and cut off past its limit (the range asked for, or
+    `maxDownload`): a server cannot make the page buffer more.
+  - Fixed block size (`blockSize`, 1 MiB by default); a paged file's page
+    size is not used. No retries: a failed request fails the call (calling
+    again retries it; what was fetched stays cached).
+  - Tested under Node 22 and headless Chromium (Playwright's build) against
+    a local server, cross-origin included (a page on 127.0.0.1 reading a
+    file from localhost, with and without exposed headers); not in Firefox
+    or Safari.
+  - The native corpus comparison (`tests/lazy.rs` with
+    `CLAWHDF5_WASM_CORPUS`) fails now and then on one CVE file,
+    `cve-2025-2310.h5`: two of its datasets have more than one bad chunk,
+    and which chunk's error is reported depends on the iteration order of
+    the chunk index (a `HashMap`, seeded per process), so the lazy and
+    the range-storage reads can name different errors. Both are errors;
+    not specific to `openUrl` (it predates it).
 - Compound, reference, opaque, bitfield, time and VL-sequence datasets are
   refused with an error naming the type; attributes of those types come back
   as `value: null` with their `dtype`.
diff --git a/examples/wasm-viewer/README.md b/examples/wasm-viewer/README.md
index 1a5c47d..dc9dc05 100644
--- a/examples/wasm-viewer/README.md
+++ b/examples/wasm-viewer/README.md
@@ -2,10 +2,12 @@
 
 A single page that opens an HDF5 or NetCDF-4 file entirely in the browser
 with `clawhdf5-wasm` (clawhdf5's reader compiled to WebAssembly): drop a
-file, browse its groups, and look at a dataset's type, shape, attributes
-and values (a 50 x 12 window at a time, read as a hyperslab, with the
-leading dimensions of a 3-D+ dataset held at chosen indices). The file never
-leaves the page.
+file or give a URL, browse its groups, and look at a dataset's type, shape,
+attributes and values (a 50 x 12 window at a time, read as a hyperslab,
+with the leading dimensions of a 3-D+ dataset held at chosen indices). A
+local file never leaves the page. A file given by URL is not downloaded:
+only the byte ranges each view needs are fetched (HTTP range requests),
+and the header shows the requests and bytes that has cost so far.
 
 ## Build and open
 
@@ -16,14 +18,22 @@ bash examples/wasm-viewer/build.sh                  # writes examples/wasm-viewe
 python3 -m http.server -d examples/wasm-viewer 8000 # wasm cannot load from file://
 ```
 
-Then open <http://localhost:8000/>. `?file=<url>&path=<object>` opens a
-file from a URL (same origin, or one serving CORS headers) and selects an
-object in it, e.g. `?file=data/run1.h5&path=/results/energy`.
+Then open <http://localhost:8000/>. The URL box, or
+`?file=<url>&path=<object>`, opens a file by URL (same origin, or a server
+sending CORS headers, see Limits) and selects an object in it, e.g.
+`?file=data/run1.h5&path=/results/energy`. Python's `http.server` does not
+answer range requests, so a file served by it is downloaded whole (the
+header says so); `test/serve.py` does:
+
+```bash
+python3 examples/wasm-viewer/test/serve.py --root data=/path/to/files --root =examples/wasm-viewer
+# prints its port; open http://127.0.0.1:<port>/?file=data/run1.h5
+```
 
 ## JavaScript API
 
 ```js
-import init, { open } from "./pkg/clawhdf5_wasm.js";
+import init, { open, openUrl } from "./pkg/clawhdf5_wasm.js";
 await init();
 const f = open(new Uint8Array(await blob.arrayBuffer()));
 f.list("/");                          // [{ name, kind: "group" | "dataset" }], groups first
@@ -32,8 +42,41 @@ f.attrs("/grid");                     // [{ name, value, dtype }]
 f.read("/grid");                      // { shape, dtype, data }
 f.readHyperslab("/grid", [0, 0], [10, 5], [2, 1]);  // start, count, stride?, block?
 f.free();
+
+// A file on a web server, read by HTTP range requests as needed. The same
+// methods, each returning a promise.
+const r = await openUrl("https://example.org/run1.h5", { blockSize: 1 << 20 });
+await r.list("/");
+await r.readHyperslab("/grid", [0, 0], [10, 5]);  // fetches only the chunks it touches
+r.stats();    // { lazy, size, requests, bytesFetched, cachedBytes, passes }
+r.free();
 ```
 
+`openUrl(url, opts)` options, all optional: `blockSize` (bytes per block
+fetched, 512 B to 64 MiB, default 1 MiB), `cacheSize` (bytes of blocks kept
+between calls, default 64 MiB), `maxFetch` (bytes one call may fetch, and
+so the longest single read, up to 1 GiB, default 512 MiB), `fallback`
+(`"download"`, the default, reads the whole file when the server ignores
+`Range`, up to `maxDownload` bytes, default 512 MiB, at most 1 GiB;
+`"error"` refuses such a server), `headers` (a `Headers`, `[name, value]`
+pairs or an object) and `credentials` (passed to every request),
+`parallel` (requests in flight, 1 to 1024, default 6; when one fails the
+others are aborted), `fetch` (a `fetch`-compatible function to use).
+
+How it works: the reader is synchronous and a page cannot block on the
+network, so each call runs as a *pass* over the blocks fetched so far. A
+pass that needs a block not yet fetched is abandoned, the missing blocks
+are fetched (in parallel, adjacent blocks in one request), and the pass is
+run again, until one completes (`docs/design/range-reads.md`, M4). Opening
+costs one request (the first block, which also gives the file's size);
+listing a group whose metadata is in blocks already fetched costs none,
+and otherwise a round trip per level of the group's index plus one for
+its children's headers, all fetched together;
+reading a chunked dataset costs a round trip for its chunk index (a few
+for a deep one) and one batch of requests for its chunks. Every answer is
+checked — a `206` with exactly the bytes asked for, from the same file
+(ETag or Last-Modified, and length) — or the call fails.
+
 `data` is the typed array of the stored width (`Float64Array`,
 `Float32Array` also for `f16`, `Int8Array` ... `BigInt64Array`,
 `BigUint64Array`), or an array of strings for fixed- and variable-length
@@ -43,7 +86,17 @@ else throws an `Error` naming the type.
 
 ## Limits
 
-- Read-only, and the whole file is held in memory (no range requests).
+- Read-only. `open(bytes)` holds the whole file in memory.
+- `openUrl`: a cross-origin server must allow CORS and expose
+  `Content-Range` (`Access-Control-Expose-Headers: Content-Range`) or answer
+  `HEAD` with `Content-Length`; if the page cannot see `ETag` or
+  `Last-Modified` either, a file replaced on the server is detected only by
+  a change of length. A call keeps what it reads until it finishes, and
+  may fetch at most `maxFetch`; `read()` of a dataset that would take more
+  than 1 GiB to decode is refused (read windows of large datasets with
+  `readHyperslab`). Files of 4 GiB or more are refused at open (wasm32).
+  Every response body is cut off past the length asked for. More in
+  `docs/known-issues.md`.
 - Compound, reference, opaque and variable-length-sequence datasets are
   refused with an error. Attributes of those types are listed with
   `value: null` and their `dtype`.
@@ -56,28 +109,55 @@ else throws an `Error` naming the type.
 ## Tests
 
 `test/run.sh` builds the package, writes `fixture.h5` (h5py) and
-`fixture.nc` (netCDF4) with `test/make_fixture.py`, then:
+`fixture.nc` (netCDF4) with `test/make_fixture.py`, and `big.h5`, a 200 MB
+h5py file (`WASM_BIG_MB` sets its size, 0 leaves it out; it goes under
+`TMPDIR`), serves them with `test/serve.py` (range requests, a request
+counter, `/norange/...` for a server without range support,
+`/noexpose/...` and `/unexposed/...` for one that does not expose its
+headers to CORS), then:
 
 - runs `test/test.mjs` under Node: every dataset (whole and a strided
   hyperslab), listing and attribute is compared with what libhdf5 reads
   back, error paths are checked, and so are the page's DOM-free helpers
-  (`viewer-lib.js`);
+  (`viewer-lib.js`). Then the same checks on the files opened by URL (1 MiB
+  and 512 B blocks), calls in flight at once, the request budget of
+  `big.h5` (listing it and reading small things of it must take at most 8
+  requests and under 5% of the file), the download fallback and its
+  limit, and the errors: HTTP status, a file that changes, a server that
+  sends the wrong bytes or stops honouring `Range`. Also the cross-origin
+  path with no exposed headers (`/noexpose/`: length from `HEAD`), the
+  size limits on `make_fixture.py`'s `limits.h5`, `hostile_vl.h5` (a heap
+  collection claiming 2 GiB) and `far.h5` (data at 3 GiB, served by a
+  mock), bodies longer than asked for, `headers` forms, `parallel`, and
+  sibling requests aborted after a failure.
+  `CLAWHDF5_WASM_CORPUS=DIR` also compares every HDF5 file under `DIR` (up
+  to 16 MiB) read by URL with the same file read from bytes;
 - runs `test/browser.sh`: loads the page in headless Chromium with
-  `?file=fixture.h5&path=...` for eight objects and checks the rendered tree,
-  types, shapes, attribute and value cells, and the error shown for an
-  unsupported type. Skipped when no Chromium is found (`CHROME` names one;
-  a Playwright download under `~/.cache/ms-playwright` is picked up).
-  Drag-and-drop and the file picker are not driven by it; they share
-  `load()` with the `?file=` path.
+  `?file=fix/fixture.h5&path=...` for eight objects and checks the rendered
+  tree, types, shapes, attribute and value cells, the request counter, and
+  the error shown for an unsupported type; then the file from another
+  origin (localhost), with CORS exposing `Content-Range` and exposing
+  nothing (`/unexposed/`, where the server must see a `HEAD`), a server
+  without range support, and `big.h5` (a small dataset and a window of the large one,
+  with a single-digit percentage of the file fetched). Skipped when no
+  Chromium is found (`CHROME` names one; a Playwright download under
+  `~/.cache/ms-playwright` is picked up). Drag-and-drop, the file picker
+  and the URL box are not driven by it; they share `setFile()` with the
+  `?file=` path.
 
 The same expectations are checked natively, without Node, by
-`crates/clawhdf5-wasm/tests/h5py_interop.rs`, which is what CI runs (the CI
-container has no Node or browser).
+`crates/clawhdf5-wasm/tests/h5py_interop.rs`, and the lazy reader against
+the in-memory one by `tests/lazy.rs` (with `CLAWHDF5_WASM_CORPUS`, over the
+corpus too), which is what CI runs (the CI container has no Node or
+browser).
 
 ## Size
 
 Measured 2026-09-26 on tank (rustc 1.98.1, wasm-bindgen 0.2.129, gzip 1.14,
-`gzip -9 -n`), after `bash examples/wasm-viewer/build.sh`:
+`gzip -9 -n`), after `bash examples/wasm-viewer/build.sh`. The package is
+larger now and the table has not been re-measured: the reader has grown
+since, and `openUrl` (2026-09-27) made the facade's range-read path
+reachable from JavaScript and added promise glue and `remote.js`.
 
 | | raw | gzip -9 |
 |---|---:|---:|
diff --git a/examples/wasm-viewer/index.html b/examples/wasm-viewer/index.html
index d477e49..279c9b8 100644
--- a/examples/wasm-viewer/index.html
+++ b/examples/wasm-viewer/index.html
@@ -56,27 +56,39 @@
                     border: 1px solid var(--line); border-radius: 4px; }
   .error { color: var(--bad); font-family: var(--mono); white-space: pre-wrap; }
   .muted { color: var(--muted); }
+  form.url { display: flex; gap: 6px; margin-left: auto; flex: 1 1 320px; max-width: 560px; }
+  form.url input { flex: 1; min-width: 0; font: 13px var(--mono); padding: 4px 8px; background: var(--panel);
+                   color: var(--ink); border: 1px solid var(--line); border-radius: 6px; }
+  .netstats { flex-basis: 100%; color: var(--muted); font-size: 12.5px; font-family: var(--mono); }
+  .netstats:empty { display: none; }
 </style>
 </head>
 <body>
 <header>
   <h1>HDF5 Viewer</h1>
   <span class="file" id="filename">no file</span>
+  <form class="url" id="urlform">
+    <input type="url" id="url" placeholder="https://…/file.h5 (read by HTTP range requests)" aria-label="File URL">
+    <button class="button" type="submit">Open URL</button>
+  </form>
   <label class="button">Open file…<input type="file" id="picker" accept=".h5,.hdf5,.he5,.nc,.nc4,.cdf" hidden></label>
+  <span class="netstats" id="netstats"></span>
 </header>
 <main>
   <nav id="tree"></nav>
   <section id="detail">
     <div class="drop">
-      <p><strong>Drop an HDF5 or NetCDF-4 file here</strong>, or use “Open file…”.</p>
-      <p class="muted">The file is read in this page by clawhdf5 compiled to WebAssembly; it is not uploaded anywhere.</p>
+      <p><strong>Drop an HDF5 or NetCDF-4 file here</strong>, use “Open file…”, or give a URL.</p>
+      <p class="muted">The file is read in this page by clawhdf5 compiled to WebAssembly; a local file is not uploaded anywhere.
+        A file given by URL is not downloaded: only the byte ranges each view needs are fetched (HTTP range requests),
+        and the requests and bytes it has cost are shown at the top.</p>
       <p class="muted" id="version"></p>
     </div>
   </section>
 </main>
 <script type="module">
-import init, { open, version } from "./pkg/clawhdf5_wasm.js";
-import { joinPath, formatValue, viewWindow, toRows, perElement } from "./viewer-lib.js";
+import init, { open, openUrl, version } from "./pkg/clawhdf5_wasm.js";
+import { joinPath, formatValue, viewWindow, toRows, perElement, formatStats } from "./viewer-lib.js";
 
 const $ = (id) => document.getElementById(id);
 const el = (tag, props = {}, ...kids) => {
@@ -85,30 +97,58 @@ const el = (tag, props = {}, ...kids) => {
   return e;
 };
 
+// The open file: an H5File (a local file, read in memory; synchronous
+// methods) or a RemoteFile (openUrl: bytes fetched by range requests as
+// needed; methods return promises). Every call below is awaited, so both
+// work the same.
 let file = null;
 let selectedNode = null;
+// Bumped by every selection, so a slow read for an object no longer shown
+// does not overwrite the current one.
+let showSeq = 0;
 
 await init();
 $("version").textContent = `clawhdf5 ${version()}`;
 
-async function load(blob) {
-  const bytes = new Uint8Array(await blob.arrayBuffer());
+// Requests and bytes a remote file has cost so far.
+function updateStats() {
+  $("netstats").textContent = file && file.stats ? formatStats(file.stats()) : "";
+}
+
+async function setFile(name, opener) {
   if (file) file.free();
   file = null;
-  $("filename").textContent = blob.name;
+  selectedNode = null;
+  showSeq++;
+  $("filename").textContent = name;
   $("tree").replaceChildren();
+  $("netstats").textContent = "";
+  $("detail").replaceChildren(el("p", { className: "muted", textContent: `Opening ${name}…` }));
   try {
-    file = open(bytes);
+    file = await opener();
   } catch (e) {
-    $("detail").replaceChildren(el("p", { className: "error", textContent: `Cannot open ${blob.name}: ${e.message}` }));
+    $("detail").replaceChildren(el("p", { className: "error", textContent: `Cannot open ${name}: ${e.message}` }));
     return;
   }
+  updateStats();
   const root = el("ul", { className: "tree" });
   root.append(treeNode("/", "/", "group"));
   $("tree").append(root);
-  root.querySelector(".node").click();
+  await activate(root.querySelector(".node"));
 }
 
+async function load(blob) {
+  const bytes = new Uint8Array(await blob.arrayBuffer());
+  await setFile(blob.name, async () => open(bytes));
+}
+
+async function loadUrl(url) {
+  $("url").value = url;
+  await setFile(url, () => openUrl(new URL(url, location.href).href));
+}
+
+// A tree row: clicking it selects it (and expands a group); `activate`
+// does the same and resolves once the listing and the detail are shown.
 function treeNode(path, name, kind) {
   const li = el("li");
   const icon = el("span", { className: "icon", textContent: kind === "group" ? "▸" : "·" });
@@ -116,34 +156,39 @@ function treeNode(path, name, kind) {
   row.dataset.path = path;
   li.append(row);
   let children = null;
-  row.addEventListener("click", () => {
+  row.activate = async () => {
     if (selectedNode) selectedNode.classList.remove("selected");
     row.classList.add("selected");
     selectedNode = row;
+    const shown = show(path, kind);
     if (kind === "group") {
       if (children) {
         children.hidden = !children.hidden;
       } else {
         children = el("ul", { className: "tree" });
+        li.append(children);
         try {
-          for (const c of file.list(path)) children.append(treeNode(joinPath(path, c.name), c.name, c.kind));
+          for (const c of await file.list(path)) children.append(treeNode(joinPath(path, c.name), c.name, c.kind));
         } catch (e) {
           children.append(el("li", { className: "error", textContent: e.message }));
         }
-        li.append(children);
+        updateStats();
       }
       icon.textContent = children.hidden ? "▸" : "▾";
     }
-    show(path, kind);
-  });
+    await shown;
+  };
+  row.addEventListener("click", () => row.activate());
   return li;
 }
 
-function attrsTable(path) {
+const activate = (row) => row.activate();
+
+async function attrsTable(path) {
   let attrs, errors;
   try {
-    attrs = file.attrs(path);
-    errors = file.attrErrors(path);
+    attrs = await file.attrs(path);
+    errors = await file.attrErrors(path);
   } catch (e) {
     return el("p", { className: "error", textContent: e.message });
   }
@@ -157,14 +202,16 @@ function attrsTable(path) {
   return el("div", { className: "scroll" }, t);
 }
 
-function show(path, kind) {
+async function show(path, kind) {
+  const seq = ++showSeq;
+  const current = () => seq === showSeq;
   const out = [el("h2", { textContent: path })];
   if (kind === "dataset") {
     let info;
     try {
-      info = file.info(path);
+      info = await file.info(path);
     } catch (e) {
-      $("detail").replaceChildren(...out, el("p", { className: "error", textContent: e.message }));
+      if (current()) $("detail").replaceChildren(...out, el("p", { className: "error", textContent: e.message }));
       return;
     }
     const max = info.maxshape === null ? "—" : `(${info.maxshape.map((d) => d ?? "∞").join(", ")})`;
@@ -172,15 +219,16 @@ function show(path, kind) {
       el("dt", { textContent: "type" }), el("dd", { textContent: info.dtype }),
       el("dt", { textContent: "shape" }), el("dd", { textContent: `(${info.shape.join(", ")})` }),
       el("dt", { textContent: "max shape" }), el("dd", { textContent: max })));
-    out.push(el("h3", { textContent: "Attributes" }), attrsTable(path));
-    out.push(el("h3", { textContent: "Values" }), valuesView(path, info));
+    out.push(el("h3", { textContent: "Attributes" }), await attrsTable(path));
+    out.push(el("h3", { textContent: "Values" }), await valuesView(path, info, current));
   } else {
-    out.push(el("h3", { textContent: "Attributes" }), attrsTable(path));
+    out.push(el("h3", { textContent: "Attributes" }), await attrsTable(path));
   }
-  $("detail").replaceChildren(...out);
+  updateStats();
+  if (current()) $("detail").replaceChildren(...out);
 }
 
-function valuesView(path, info) {
+async function valuesView(path, info, current) {
   const shape = info.shape;
   const per = perElement(info.elementShape);
   const state = { row: 0, col: 0, rows: 50, cols: 12, fixed: shape.slice(0, Math.max(0, shape.length - 2)).map(() => 0) };
@@ -202,15 +250,19 @@ function valuesView(path, info) {
   if (shape.length >= 2) controls.append(num("column", "col"));
   state.fixed.forEach((_, i) => controls.append(num(`dim ${i}`, "fixed", i)));
 
-  function render() {
+  let renderSeq = 0;
+  async function render() {
+    const seq = ++renderSeq;
     const w = viewWindow(shape, state);
     let res;
     try {
-      res = w === null ? file.read(path) : file.readHyperslab(path, w.start, w.count);
+      res = w === null ? await file.read(path) : await file.readHyperslab(path, w.start, w.count);
     } catch (e) {
-      body.replaceChildren(el("p", { className: "error", textContent: e.message }));
+      if (seq === renderSeq) body.replaceChildren(el("p", { className: "error", textContent: e.message }));
       return;
     }
+    updateStats();
+    if (seq !== renderSeq || !current()) return;
     if (w === null) {
       body.replaceChildren(el("pre", { textContent: toRows(res.data, 1, 1, per)[0][0] }));
       return;
@@ -229,41 +281,39 @@ function valuesView(path, info) {
       (shape.length >= 2 ? `, columns ${w.col}–${w.col + w.cols - 1}` : "") + ` of ${total} values` });
     body.replaceChildren(t, note);
   }
-  render();
+  await render();
   return box;
 }
 
 // Expand the tree down to `path` and select it.
-function reveal(path) {
+async function reveal(path) {
   const rowFor = (p) => [...document.querySelectorAll(".node")].find((n) => n.dataset.path === p);
   let cur = "/";
   for (const part of path.split("/").filter(Boolean)) {
     const row = rowFor(cur);
     if (!row) return;
     const kids = row.parentElement.querySelector(":scope > ul");
-    if (!kids || kids.hidden) row.click();
+    if (!kids || kids.hidden) await activate(row);
     cur = joinPath(cur, part);
   }
   const target = rowFor(cur);
-  if (target && target !== selectedNode) target.click();
+  if (target && target !== selectedNode) await activate(target);
 }
 
 // ?file=<url>&path=<object> opens a file from a URL (same origin, or one
-// that allows CORS) and selects an object in it.
+// that allows CORS) by range requests and selects an object in it.
 const params = new URLSearchParams(location.search);
 if (params.get("file")) {
-  const url = params.get("file");
-  try {
-    const resp = await fetch(url);
-    if (!resp.ok) throw new Error(`HTTP ${resp.status}`);
-    const blob = await resp.blob();
-    await load(new File([blob], url.split("/").pop()));
-    if (file && params.get("path")) reveal(params.get("path"));
-  } catch (e) {
-    $("detail").replaceChildren(el("p", { className: "error", textContent: `Cannot fetch ${url}: ${e.message}` }));
-  }
+  await loadUrl(params.get("file"));
+  if (file && params.get("path")) await reveal(params.get("path"));
+  document.body.dataset.ready = "1";
 }
 
+$("urlform").addEventListener("submit", (e) => {
+  e.preventDefault();
+  const url = $("url").value.trim();
+  if (url) loadUrl(url);
+});
 $("picker").addEventListener("change", (e) => e.target.files[0] && load(e.target.files[0]));
 document.addEventListener("dragover", (e) => { e.preventDefault(); document.body.classList.add("dragging"); });
 document.addEventListener("dragleave", () => document.body.classList.remove("dragging"));
diff --git a/examples/wasm-viewer/test/browser.sh b/examples/wasm-viewer/test/browser.sh
index a9ea5aa..df31f7f 100644
--- a/examples/wasm-viewer/test/browser.sh
+++ b/examples/wasm-viewer/test/browser.sh
@@ -3,10 +3,13 @@
 #
 #   browser.sh FIXTURE_DIR
 #
-# FIXTURE_DIR holds fixture.h5 from make_fixture.py; ../pkg must be built.
-# The page is opened with ?file=fixture.h5&path=<object>, which fetches the
-# file, builds the tree down to <object> and shows it; the rendered DOM is
-# dumped and checked for the values libhdf5 reads.
+# FIXTURE_DIR holds fixture.h5 from make_fixture.py (and big.h5 when it was
+# run with WASM_BIG_MB); ../pkg must be built. test/serve.py serves the
+# page and, under /fix, the fixtures, with HTTP range requests (no copies,
+# no links). The page is opened with ?file=fix/<name>&path=<object>, which
+# opens the file with openUrl (range requests), builds the tree down to
+# <object> and shows it; the rendered DOM is dumped and checked for the
+# values libhdf5 reads and for the request counter.
 #
 # Browser: $CHROME, else chromium/google-chrome on PATH, else a Playwright
 # download under ~/.cache/ms-playwright. Exit 3 when none is found.
@@ -31,63 +34,90 @@ if [ -z "$chrome" ] || [ ! -x "$chrome" ]; then
     exit 3
 fi
 
-root="$(mktemp -d)"
+profiles="$(mktemp -d)"
 server=""
 cleanup() {
     [ -n "$server" ] && kill "$server" 2>/dev/null || true
-    rm -rf "$root"
+    rm -rf "$profiles"
 }
 trap cleanup EXIT
-ln -s "$HERE/../index.html" "$HERE/../viewer-lib.js" "$HERE/../pkg" "$FIX/fixture.h5" "$root/"
 
-port=$("$PY" -c 'import socket; s = socket.socket(); s.bind(("127.0.0.1", 0)); print(s.getsockname()[1])')
-"$PY" -m http.server --bind 127.0.0.1 --directory "$root" "$port" >/dev/null 2>&1 &
+"$PY" "$HERE/serve.py" --root "fix=$FIX" --root "=$HERE/.." > "$profiles/port" &
 server=$!
 for _ in $(seq 50); do
-    "$PY" -c "import urllib.request; urllib.request.urlopen('http://127.0.0.1:$port/index.html')" 2>/dev/null && break
+    [ -s "$profiles/port" ] && break
     sleep 0.1
 done
+port="$(head -1 "$profiles/port")"
 
 fails=0
 # A fresh profile per page: a second instance on the same profile fails.
 render() {
     local profile
-    profile="$(mktemp -d "$root/profile.XXXXXX")"
+    profile="$(mktemp -d "$profiles/profile.XXXXXX")"
     "$chrome" --headless --no-sandbox --disable-gpu --user-data-dir="$profile" \
         --virtual-time-budget=20000 \
-        --dump-dom "http://127.0.0.1:$port/index.html?file=fixture.h5&path=$1" 2>/dev/null
+        --dump-dom "http://127.0.0.1:$port/index.html?file=$1&path=$2" 2>/dev/null
 }
-# expect PATH TEXT...: every TEXT appears in the page rendered for PATH.
+# expect FILE PATH TEXT...: every TEXT appears in the page rendered for PATH
+# of FILE (a URL relative to the page); "re:TEXT" is an extended regex.
 expect() {
-    local path="$1" dom
-    shift
-    dom="$(render "$path")"
-    [ -n "${BROWSER_DEBUG:-}" ] && printf "%s\n" "$dom" > "$BROWSER_DEBUG.$(echo "$path" | tr / _).html"
+    local file="$1" path="$2" dom
+    shift 2
+    dom="$(render "$file" "$path")"
+    [ -n "${BROWSER_DEBUG:-}" ] && printf "%s\n" "$dom" > "$BROWSER_DEBUG.$(echo "$file$path" | tr / _).html"
     for text in "$@"; do
-        if ! grep -qF -- "$text" <<<"$dom"; then
-            echo "FAIL: page for $path lacks: $text" >&2
+        local flag=-qF
+        if [ "${text#re:}" != "$text" ]; then flag=-qE; text="${text#re:}"; fi
+        if ! grep $flag -- "$text" <<<"$dom"; then
+            echo "FAIL: page for $file $path lacks: $text" >&2
             fails=$((fails + 1))
         fi
     done
-    echo "rendered $path"
+    echo "rendered $file $path"
 }
 
+H5=fix/fixture.h5
 # Tree (root expanded; the group row carries its path) and root attributes.
-expect "/" 'data-path="/sensors"' 'data-path="/grid"' '<td>title</td><td>"wasm fixture"</td>' \
+expect $H5 "/" 'data-path="/sensors"' 'data-path="/grid"' '<td>title</td><td>"wasm fixture"</td>' \
     '<td>big</td><td>9223372036854775813</td>' '(compound{x: f64, n: i32})'
-# A chunked, deflated 2-D dataset: type, shape, the first window of values.
-expect "/grid" '<dd>f64</dd>' '<dd>(6, 10)</dd>' '<th>9</th>' '<td>0.25</td>' '<td>14.75</td>' \
-    'showing rows 0–5, columns 0–9 of 60 values'
+# A chunked, deflated 2-D dataset: type, shape, the first window of values;
+# the request counter.
+expect $H5 "/grid" '<dd>f64</dd>' '<dd>(6, 10)</dd>' '<th>9</th>' '<td>0.25</td>' '<td>14.75</td>' \
+    'showing rows 0–5, columns 0–9 of 60 values' 'request' 'fetched (100.0%)'
 # Nested path revealed through the tree; big-endian float32.
-expect "/sensors/temp" 'data-path="/sensors/temp"' '<dd>f32</dd>' '<td>21.5</td>' '<td>22.25</td>'
+expect $H5 "/sensors/temp" 'data-path="/sensors/temp"' '<dd>f32</dd>' '<td>21.5</td>' '<td>22.25</td>'
 # 64-bit integers stay exact; strings; array datatype cells.
-expect "/u64" '<td>18446744073709551615</td>'
-expect "/vlen_str" '<td>"двa"</td>' '<dd>vlen string</dd>'
-expect "/pairs" '<td>[2, 3]</td>' '<dd>array[2]&lt;i32&gt;</dd>'
+expect $H5 "/u64" '<td>18446744073709551615</td>'
+expect $H5 "/vlen_str" '<td>"двa"</td>' '<dd>vlen string</dd>'
+expect $H5 "/pairs" '<td>[2, 3]</td>' '<dd>array[2]&lt;i32&gt;</dd>'
 # 3-D: leading dimension held at 0, window over the last two.
-expect "/cube" '<dd>(2, 5, 6)</dd>' '<td>29</td>' 'dim 0'
+expect $H5 "/cube" '<dd>(2, 5, 6)</dd>' '<td>29</td>' 'dim 0'
 # Unsupported type: an error, not values.
-expect "/table" 'class="error"' 'reading compound{x: f64, n: i32} datasets is not supported'
+expect $H5 "/table" 'class="error"' 'reading compound{x: f64, n: i32} datasets is not supported'
+# Cross-origin: the page is on 127.0.0.1, the file on localhost. CORS that
+# exposes Content-Range, and CORS that exposes nothing (/unexposed/: the
+# browser hides Content-Range and ETag, so the page learns the length with
+# a HEAD request, which the server's log must show).
+expect "http://localhost:$port/$H5" "/grid" '<td>14.75</td>' 'request'
+get() { "$PY" -c 'import sys, urllib.request; print(urllib.request.urlopen(sys.argv[1]).read().decode())' "$1"; }
+get "http://127.0.0.1:$port/__reset" >/dev/null
+expect "http://localhost:$port/unexposed/$H5" "/grid" '<td>0.25</td>' '<td>14.75</td>' 'request'
+heads="$(get "http://127.0.0.1:$port/__stats" | "$PY" -c \
+    'import json, sys; print(sum(1 for l in json.load(sys.stdin)["log"] if l[4] == "HEAD" and l[0].endswith("fixture.h5")))')"
+if [ "$heads" -lt 1 ]; then
+    echo "FAIL: no HEAD request for the length with unexposed headers" >&2
+    fails=$((fails + 1))
+fi
+# A server without range support: the file is downloaded whole, and says so.
+expect norange/$H5 "/grid" '<td>14.75</td>' 'downloaded whole'
+# A large file: a small dataset, and a window of the large one, fetch a few
+# blocks (the counter shows a small percentage: "(1.5%)", not "(100.0%)").
+if [ -f "$FIX/big.h5" ]; then
+    expect fix/big.h5 "/small" '<td>1.5</td>' '<td>3.25</td>' 're:MiB of 191 MiB fetched \([0-9]\.[0-9]%\)'
+    expect fix/big.h5 "/big" '<dd>(25000000)</dd>' '<td>0</td>' '<td>24.5</td>' \
+        're:MiB of 191 MiB fetched \([0-9]\.[0-9]%\)'
+fi
 
 if [ "$fails" -gt 0 ]; then
     echo "browser: $fails checks failed" >&2
diff --git a/examples/wasm-viewer/test/make_fixture.py b/examples/wasm-viewer/test/make_fixture.py
index 8c7722c..2a2af72 100644
--- a/examples/wasm-viewer/test/make_fixture.py
+++ b/examples/wasm-viewer/test/make_fixture.py
@@ -3,7 +3,9 @@ reads back from them, for the clawhdf5-wasm tests.
 
     python make_fixture.py OUT_DIR
 
-writes OUT_DIR/fixture.h5, OUT_DIR/fixture.nc and OUT_DIR/expected.json.
+writes OUT_DIR/fixture.h5, OUT_DIR/fixture.nc and OUT_DIR/expected.json,
+and the limit-test files OUT_DIR/limits.h5 and OUT_DIR/hostile_vl.h5 (see
+write_limits).
 Both the Rust test (crates/clawhdf5-wasm/tests/h5py_interop.rs, native) and
 the Node test (test.mjs, the built wasm package) compare against the same
 expected.json, so the two check the same values.
@@ -14,6 +16,7 @@ encoded as strings so JSON.parse keeps 64-bit values exact.
 """
 
 import json
+import os
 import sys
 import warnings
 from pathlib import Path
@@ -201,3 +204,105 @@ def slab_for(obj):
 
 json.dump({"fixture.h5": describe(h5), "fixture.nc": describe(nc)},
           open(out / "expected.json", "w"), indent=1, ensure_ascii=False)
+
+
+HUGE_U8 = 2**28 + 1024
+# The collection size hostile_vl.h5 claims: past 2 GiB, which a wasm32
+# buffer cannot hold.
+HOSTILE_GCOL_SIZE = 2**31 + 4096
+# The file length a server claims for hostile_vl.h5 (the tests' mock fetch
+# answers every range with zeros past the real bytes): 3 GiB, within what
+# wasm32 opens, and room for the collection.
+HOSTILE_LENGTH = 3 << 30
+
+
+def write_limits(out):
+    """Files for the size limits (the tests must get errors, not aborts):
+
+    - limits.h5: /huge_u8, 2^28 + 1024 bytes of u8 in compressed chunks
+      (a small file): read whole it would take over 2 GiB while decoding;
+      its last value is 7.
+    - hostile_vl.h5: a variable-length string dataset /a whose global heap
+      collection claims HOSTILE_GCOL_SIZE bytes, with the superblock's end
+      of file set to HOSTILE_LENGTH (libhdf5 cannot read it; it is only
+      served by a mock that claims that length).
+    - far.h5 and far.json: /x, 16 float64 values, whose contiguous data
+      address is moved FAR_SHIFT bytes on (past 2 GiB, the sign bit of a
+      wasm32 isize) in a file whose end of file is moved as far; the tests'
+      mock serves the data there, to show offsets up to 4 GiB work on
+      wasm32.
+    """
+    with h5py.File(out / "limits.h5", "w") as f:
+        d = f.create_dataset("huge_u8", shape=(HUGE_U8,), dtype="u1",
+                             chunks=(1 << 20,), compression="gzip")
+        d[-1] = 7
+    path = out / "hostile_vl.h5"
+    with h5py.File(path, "w", libver="earliest") as f:
+        f.create_dataset("a", data=["x", "yy"], dtype=h5py.string_dtype())
+    b = bytearray(path.read_bytes())
+    assert b[8] == 0, "a version 0 superblock"
+    b[40:48] = HOSTILE_LENGTH.to_bytes(8, "little")  # end of file address
+    at = b.index(b"GCOL")
+    b[at + 8:at + 16] = HOSTILE_GCOL_SIZE.to_bytes(8, "little")
+    path.write_bytes(bytes(b))
+
+    path = out / "far.h5"
+    values = np.arange(16, dtype="<f8") * 1.5
+    with h5py.File(path, "w", libver="earliest") as f:
+        f.create_dataset("x", data=values)
+        data_at = f["x"].id.get_offset()
+    b = bytearray(path.read_bytes())
+    # The layout message: the data's address, then its size.
+    old = data_at.to_bytes(8, "little") + (values.nbytes).to_bytes(8, "little")
+    assert b.count(old) == 1
+    at = b.index(old)
+    b[at:at + 8] = (data_at + FAR_SHIFT).to_bytes(8, "little")
+    length = len(b) + FAR_SHIFT
+    b[40:48] = length.to_bytes(8, "little")
+    path.write_bytes(bytes(b))
+    json.dump({"data_at": data_at, "far_at": data_at + FAR_SHIFT,
+               "nbytes": values.nbytes, "length": length,
+               "values": [float(x) for x in values]},
+              open(out / "far.json", "w"))
+
+
+# far.h5's data moves this far: past 2 GiB, below 4 GiB.
+FAR_SHIFT = 3 << 30
+
+write_limits(out)
+
+
+def write_big(path, megabytes):
+    """A large file for the range-request tests (`openUrl`): `/big`, about
+    `megabytes` MB of float64 in 1 MiB chunks, written after a small
+    dataset and a group, so listing and reading `/small` touch a few blocks
+    of the file and a window of `/big` one chunk. Returns what h5py reads
+    back."""
+    n = megabytes * 1_000_000 // 8
+    chunk = 1 << 17
+    with h5py.File(path, "w") as f:
+        f.attrs["note"] = "large file for range reads"
+        f.create_dataset("small", data=np.array([1.5, -2.0, 3.25]))
+        g = f.create_group("meta")
+        g.attrs["units"] = "m"
+        g.create_dataset("ids", data=np.arange(10, dtype="<i4"))
+        big = f.create_dataset("big", shape=(n,), dtype="<f8", chunks=(chunk,))
+        for s in range(0, n, 1 << 22):
+            e = min(n, s + (1 << 22))
+            big[s:e] = np.arange(s, e, dtype="<f8") * 0.5
+    start = n // 2 + 12_345
+    with h5py.File(path, "r") as f:
+        return {
+            "size": path.stat().st_size,
+            "list": {"groups": ["meta"], "datasets": ["big", "small"]},
+            "small": [float(x) for x in f["small"][()]],
+            "ids": [str(int(x)) for x in f["meta/ids"][()]],
+            "big_shape": list(f["big"].shape),
+            "window": {"start": start, "count": 10,
+                       "values": [float(x) for x in f["big"][start:start + 10]]},
+        }
+
+
+big_mb = int(os.environ.get("WASM_BIG_MB", "0"))
+if big_mb > 0:
+    json.dump(write_big(out / "big.h5", big_mb), open(out / "big.json", "w"), indent=1)
diff --git a/examples/wasm-viewer/test/run.sh b/examples/wasm-viewer/test/run.sh
index c1cdae0..68c4677 100755
--- a/examples/wasm-viewer/test/run.sh
+++ b/examples/wasm-viewer/test/run.sh
@@ -1,11 +1,17 @@
 #!/usr/bin/env bash
 # Build the wasm package (../build.sh), test it under Node against files
-# written by h5py and netCDF4 (make_fixture.py), then load the viewer page
-# in headless Chromium if one is found (browser.sh).
+# written by h5py and netCDF4 (make_fixture.py) — opened from bytes, and
+# opened by URL from a local range-capable HTTP server (serve.py) — then
+# load the viewer page in headless Chromium if one is found (browser.sh).
 #
 # Needs node, the wasm-bindgen CLI (see ../build.sh) and a Python with h5py,
 # netCDF4 and numpy: CLAWHDF5_PYTHON names it (default python3). Without that
 # Python the test is skipped, unless CLAWHDF5_REQUIRE_INTEROP=1.
+#
+# WASM_BIG_MB (default 200) sizes the large file of the range-request
+# budget test (0 leaves it out); it is written under TMPDIR.
+# CLAWHDF5_WASM_CORPUS=DIR also compares every HDF5 file under DIR (up to
+# 16 MiB) read over HTTP with the same file read from bytes.
 set -euo pipefail
 
 HERE="$(cd "$(dirname "$0")" && pwd)"
@@ -23,9 +29,24 @@ fi
 
 bash "$HERE/../build.sh"
 fix="$(mktemp -d)"
-trap 'rm -rf "$fix"' EXIT
-"$PY" "$HERE/make_fixture.py" "$fix"
-node "$HERE/test.mjs" "$HERE/../pkg" "$fix"
+server=""
+cleanup() {
+    [ -n "$server" ] && kill "$server" 2>/dev/null || true
+    rm -rf "$fix"
+}
+trap cleanup EXIT
+WASM_BIG_MB="${WASM_BIG_MB:-200}" "$PY" "$HERE/make_fixture.py" "$fix"
+
+# The fixtures (and the corpus) over HTTP with range support.
+roots=(--root "fix=$fix")
+[ -n "${CLAWHDF5_WASM_CORPUS:-}" ] && roots+=(--root "corpus=$CLAWHDF5_WASM_CORPUS")
+"$PY" "$HERE/serve.py" "${roots[@]}" > "$fix/port" &
+server=$!
+for _ in $(seq 50); do
+    [ -s "$fix/port" ] && break
+    sleep 0.1
+done
+node "$HERE/test.mjs" "$HERE/../pkg" "$fix" "http://127.0.0.1:$(head -1 "$fix/port")"
 
 # The page itself, in headless Chromium when one is available.
 status=0
diff --git a/examples/wasm-viewer/test/serve.py b/examples/wasm-viewer/test/serve.py
new file mode 100644
index 0000000..d478a14
--- /dev/null
+++ b/examples/wasm-viewer/test/serve.py
@@ -0,0 +1,223 @@
+"""A static HTTP server for the wasm tests, with HTTP Range support and
+request counting.
+
+    python serve.py [--root PREFIX=DIR ...]
+
+Serves each DIR under URL PREFIX (the first match wins; PREFIX "" is the
+site root), prints the port on its first line of stdout, and runs until
+killed. No symlinks or copies are made: files are read where they are.
+
+- `Range: bytes=a-b`, `bytes=a-` and `bytes=-n` get 206 with Content-Range,
+  an unsatisfiable range 416; every file answer carries an ETag, and CORS
+  headers exposing Content-Range, so a page on another origin can use it.
+- Under `/norange/...` the same files are served but Range is ignored
+  (200 with the whole file), as by a server without range support.
+- Under `/noexpose/...` ranges are served without Content-Range, ETag,
+  Last-Modified or Accept-Ranges, and CORS exposes none of them: what a
+  page sees of a cross-origin server that does not list them in
+  Access-Control-Expose-Headers. The length comes from Content-Length of
+  a HEAD request (always readable).
+- Under `/unexposed/...` ranges are served with all those headers, but
+  CORS exposes none of them: a browser page on another origin cannot read
+  them (test/browser.sh loads the page from 127.0.0.1 and the file from
+  localhost), so it has to take the same path.
+- `GET /__stats` returns `{"requests": n, "bytes": n, "log": [...]}` for
+  file requests since the last `GET /__reset`, which zeroes them.
+"""
+
+import argparse
+import hashlib
+import json
+import os
+import posixpath
+import sys
+import threading
+from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer
+from urllib.parse import unquote, urlsplit
+
+TYPES = {
+    ".html": "text/html; charset=utf-8",
+    ".js": "text/javascript; charset=utf-8",
+    ".mjs": "text/javascript; charset=utf-8",
+    ".wasm": "application/wasm",
+    ".json": "application/json",
+    ".ts": "text/plain; charset=utf-8",
+}
+
+lock = threading.Lock()
+stats = {"requests": 0, "bytes": 0, "log": []}
+
+
+def resolve(roots, path):
+    """The file for URL `path`, or None. `..` never leaves a root."""
+    parts = [p for p in posixpath.normpath(unquote(path)).split("/") if p]
+    if any(p in (".", "..") for p in parts):
+        return None
+    for prefix, root in roots:
+        pre = [p for p in prefix.split("/") if p]
+        if parts[: len(pre)] == pre:
+            rest = parts[len(pre):] or ["index.html"]
+            f = os.path.join(root, *rest)
+            if os.path.isfile(f):
+                return f
+    return None
+
+
+def parse_range(header, size):
+    """(start, end exclusive) for a single `bytes=` range, "bad" when
+    unsatisfiable, None when absent or unparsable (served whole)."""
+    if not header or not header.startswith("bytes=") or "," in header:
+        return None
+    a, _, b = header[len("bytes="):].strip().partition("-")
+    try:
+        if a == "":
+            n = int(b)
+            return (max(0, size - n), size) if n > 0 and size > 0 else "bad"
+        start = int(a)
+        end = int(b) + 1 if b else size
+    except ValueError:
+        return None
+    if start >= size or end <= start:
+        return "bad"
+    return start, min(end, size)
+
+
+def make_handler(roots):
+    class Handler(BaseHTTPRequestHandler):
+        protocol_version = "HTTP/1.1"
+
+        def log_message(self, *args):
+            pass
+
+        def cors(self, expose=True):
+            self.send_header("Access-Control-Allow-Origin", "*")
+            if expose:
+                self.send_header("Access-Control-Expose-Headers",
+                                 "Content-Range, Content-Length, ETag, Accept-Ranges")
+
+        def do_OPTIONS(self):
+            self.send_response(204)
+            self.cors()
+            self.send_header("Access-Control-Allow-Headers", "Range")
+            self.send_header("Content-Length", "0")
+            self.end_headers()
+
+        def do_HEAD(self):
+            self.serve(head=True)
+
+        def do_GET(self):
+            self.serve(head=False)
+
+        def json(self, obj):
+            body = json.dumps(obj).encode()
+            self.send_response(200)
+            self.cors()
+            self.send_header("Content-Type", "application/json")
+            self.send_header("Content-Length", str(len(body)))
+            self.send_header("Cache-Control", "no-store")
+            self.end_headers()
+            self.wfile.write(body)
+
+        def serve(self, head):
+            path = urlsplit(self.path).path
+            if path == "/__stats":
+                with lock:
+                    return self.json(stats)
+            if path == "/__reset":
+                with lock:
+                    stats.update(requests=0, bytes=0, log=[])
+                return self.json({})
+            # ranges: honour Range; send_all: send Content-Range, ETag and
+            # Accept-Ranges; expose: list them for CORS.
+            ranges = send_all = expose = True
+            if path.startswith("/norange/"):
+                ranges = False
+                path = path[len("/norange"):]
+            elif path.startswith("/noexpose/"):
+                expose = send_all = False
+                path = path[len("/noexpose"):]
+            elif path.startswith("/unexposed/"):
+                expose = False
+                path = path[len("/unexposed"):]
+            f = resolve(roots, path)
+            if f is None:
+                self.send_response(404)
+                self.cors()
+                self.send_header("Content-Length", "0")
+                self.end_headers()
+                return
+            size = os.path.getsize(f)
+            st = os.stat(f)
+            etag = '"%s"' % hashlib.sha1(
+                f"{f}:{size}:{st.st_mtime_ns}".encode()).hexdigest()[:16]
+            r = parse_range(self.headers.get("Range"), size) if ranges else None
+            if r == "bad":
+                self.send_response(416)
+                self.cors()
+                self.send_header("Content-Range", f"bytes */{size}")
+                self.send_header("Content-Length", "0")
+                self.end_headers()
+                return
+            start, end = r if r else (0, size)
+            self.send_response(206 if r else 200)
+            self.cors(expose)
+            ext = os.path.splitext(f)[1]
+            self.send_header("Content-Type", TYPES.get(ext, "application/octet-stream"))
+            self.send_header("Content-Length", str(end - start))
+            self.send_header("Cache-Control", "no-store")
+            if send_all:
+                self.send_header("ETag", etag)
+                if ranges:
+                    self.send_header("Accept-Ranges", "bytes")
+                if r:
+                    self.send_header("Content-Range", f"bytes {start}-{end - 1}/{size}")
+            self.end_headers()
+            with lock:
+                # A HEAD is a request too (openUrl makes one when it cannot
+                # see Content-Range); it sends no bytes.
+                stats["requests"] += 1
+                sent = 0 if head else end - start
+                stats["bytes"] += sent
+                stats["log"].append([path, start, end, 206 if r else 200, "HEAD" if head else "GET"])
+            if not head:
+                with open(f, "rb") as fh:
+                    fh.seek(start)
+                    left = end - start
+                    try:
+                        while left:
+                            buf = fh.read(min(left, 1 << 20))
+                            if not buf:
+                                break
+                            self.wfile.write(buf)
+                            left -= len(buf)
+                    except (BrokenPipeError, ConnectionResetError):
+                        pass
+
+    return Handler
+
+
+def main():
+    ap = argparse.ArgumentParser()
+    ap.add_argument("--root", action="append", default=[],
+                    help="PREFIX=DIR: serve DIR under URL PREFIX")
+    args = ap.parse_args()
+    roots = []
+    for spec in args.root:
+        prefix, _, d = spec.partition("=")
+        roots.append((prefix, os.path.abspath(d)))
+    class Server(ThreadingHTTPServer):
+        def handle_error(self, request, client_address):
+            # A client that drops a connection (a cancelled download) is
+            # not an error of the server.
+            if not isinstance(sys.exc_info()[1], (ConnectionError, TimeoutError)):
+                super().handle_error(request, client_address)
+
+    httpd = Server(("127.0.0.1", 0), make_handler(roots))
+    httpd.daemon_threads = True
+    print(httpd.server_address[1], flush=True)
+    sys.stdout.close()
+    httpd.serve_forever()
+
+
+if __name__ == "__main__":
+    main()
diff --git a/examples/wasm-viewer/test/test.mjs b/examples/wasm-viewer/test/test.mjs
index 80ca30a..e76ecdc 100644
--- a/examples/wasm-viewer/test/test.mjs
+++ b/examples/wasm-viewer/test/test.mjs
@@ -1,20 +1,31 @@
 // Node test of the built wasm package (the exact pkg/ the viewer page loads)
 // and the viewer's DOM-free helpers. Run by test/run.sh:
-//   node test.mjs PKG_DIR FIXTURE_DIR
+//   node test.mjs PKG_DIR FIXTURE_DIR [SERVER_URL]
 // FIXTURE_DIR holds fixture.h5, fixture.nc and expected.json from
-// make_fixture.py (values as libhdf5 reads them back).
+// make_fixture.py (values as libhdf5 reads them back), and big.h5/big.json
+// when it was run with WASM_BIG_MB. SERVER_URL is test/serve.py serving
+// FIXTURE_DIR under /fix (and $CLAWHDF5_WASM_CORPUS under /corpus): with
+// it, every check is repeated on files opened with openUrl (HTTP range
+// requests), and the request budget, the full-download fallback and the
+// error paths of openUrl are tested.
 import assert from "node:assert/strict";
-import { readFileSync } from "node:fs";
-import { join } from "node:path";
+import { existsSync, readFileSync, readdirSync, statSync } from "node:fs";
+import { join, relative } from "node:path";
 import { pathToFileURL } from "node:url";
 
-const [pkgDir, fixDir] = process.argv.slice(2);
+const [pkgDir, fixDir, base] = process.argv.slice(2);
 const pkg = await import(pathToFileURL(join(pkgDir, "clawhdf5_wasm.js")));
 pkg.initSync({ module: readFileSync(join(pkgDir, "clawhdf5_wasm_bg.wasm")) });
 const lib = await import(pathToFileURL(join(import.meta.dirname, "..", "viewer-lib.js")));
 
 let checks = 0;
 const eq = (a, b, msg) => { assert.deepEqual(a, b, msg); checks++; };
+// A call that must fail: a thrown Error (open) or a rejected promise
+// (openUrl), whose message matches `re`.
+const fails = async (fn, re, msg) => {
+  await assert.rejects(async () => fn(), (e) => e instanceof Error && re.test(e.message), msg);
+  checks++;
+};
 
 const ARRAY_TYPES = {
   f32: Float32Array, f64: Float64Array, i8: Int8Array, i16: Int16Array, i32: Int32Array,
@@ -54,13 +65,13 @@ function checkAttr(ctx, a, want) {
   assert.fail(`${ctx}: unknown expectation ${JSON.stringify(want)}`);
 }
 
-const expected = JSON.parse(readFileSync(join(fixDir, "expected.json"), "utf8"));
-for (const [name, exp] of Object.entries(expected)) {
-  const file = pkg.open(new Uint8Array(readFileSync(join(fixDir, name))));
-
+// Every listing, dataset, error and attribute of `file` against what
+// libhdf5 reads (expected.json). `file` is an H5File (synchronous methods)
+// or a RemoteFile (promises): every call is awaited.
+async function checkFile(name, exp, file) {
   for (const [path, want] of Object.entries(exp.lists)) {
-    eq(file.kind(path), "group", `${name}:${path} kind`);
-    const list = file.list(path);
+    eq(await file.kind(path), "group", `${name}:${path} kind`);
+    const list = await file.list(path);
     for (const [kind, key] of [["group", "groups"], ["dataset", "datasets"]]) {
       eq(list.filter((c) => c.kind === kind).map((c) => c.name).sort(), want[key], `${name}:${path} ${key}`);
     }
@@ -68,58 +79,67 @@ for (const [name, exp] of Object.entries(expected)) {
 
   for (const [path, want] of Object.entries(exp.datasets)) {
     const ctx = `${name}:${path}`;
-    eq(file.kind(path), "dataset", `${ctx} kind`);
+    eq(await file.kind(path), "dataset", `${ctx} kind`);
     if (want.unavailable) {
       // The wasm build has no zstd (it links C): a clear error, no data.
-      assert.throws(() => file.read(path), (e) => e.message.includes(want.unavailable), ctx);
-      checks++;
+      await fails(() => file.read(path), new RegExp(want.unavailable), ctx);
       continue;
     }
-    const info = file.info(path);
+    const info = await file.info(path);
     eq([...info.shape, ...info.elementShape], want.shape, `${ctx} info shape`);
-    const r = file.read(path);
+    const r = await file.read(path);
     eq(r.shape, want.shape, `${ctx} shape`);
     eq(r.dtype, info.dtype, `${ctx} dtype`);
     assert.ok(r.data instanceof ARRAY_TYPES[want.kind], `${ctx}: ${r.data.constructor.name} for ${want.kind}`);
     eq(values(want.kind, r.data), want.values, ctx);
     if (want.slab) {
       const s = want.slab;
-      const part = file.readHyperslab(path, s.start, s.count, s.stride);
+      const part = await file.readHyperslab(path, s.start, s.count, s.stride);
       eq(part.shape, s.shape, `${ctx} slab shape`);
       eq(values(want.kind, part.data), s.values, `${ctx} slab`);
     }
   }
 
   for (const [path, what] of Object.entries(exp.errors)) {
-    assert.throws(() => file.read(path), (e) => e instanceof Error && e.message.includes(what), `${name}:${path}`);
-    checks++;
+    await fails(() => file.read(path), new RegExp(what), `${name}:${path}`);
   }
 
   for (const [path, want] of Object.entries(exp.attrs)) {
-    const attrs = file.attrs(path);
-    eq(file.attrErrors(path), [], `${name}:${path} attr errors`);
+    const attrs = await file.attrs(path);
+    eq(await file.attrErrors(path), [], `${name}:${path} attr errors`);
     const seen = attrs.filter((a) => !a.name.startsWith("_") && !exp.skip_attrs.includes(a.name));
     eq(seen.map((a) => a.name).sort(), Object.keys(want).sort(), `${name}:${path} attr names`);
     for (const a of seen) checkAttr(`${name}:${path}@${a.name}`, a, want[a.name]);
   }
+}
+
+// The error paths of the reader, for an H5File or a RemoteFile of
+// fixture.h5.
+async function checkErrors(h5) {
+  await fails(() => h5.read("/nope"), /./);
+  await fails(() => h5.list("/grid"), /not a group/);
+  await fails(() => h5.readHyperslab("/grid", [0], [1]), /dimensions/);
+  await fails(() => h5.readHyperslab("/grid", [5, 0], [2, 1]), /exceeds/);
+  await fails(() => h5.readHyperslab("/grid", [-1, 0], [1, 1]), /non-negative integers/);
+  await fails(() => h5.readHyperslab("/grid", [0.5, 0], [1, 1]), /non-negative integers/);
+  // Big integers stay exact.
+  eq((await h5.read("/u64")).data[0], 18446744073709551615n, "u64 max");
+}
+
+const expected = JSON.parse(readFileSync(join(fixDir, "expected.json"), "utf8"));
+for (const [name, exp] of Object.entries(expected)) {
+  const file = pkg.open(new Uint8Array(readFileSync(join(fixDir, name))));
+  await checkFile(name, exp, file);
   file.free();
 }
 
 // Errors reach JavaScript as thrown Errors, never as data.
 const h5 = pkg.open(new Uint8Array(readFileSync(join(fixDir, "fixture.h5"))));
-const throwsMsg = (fn, re) => { assert.throws(fn, (e) => e instanceof Error && re.test(e.message)); checks++; };
-throwsMsg(() => pkg.open(new Uint8Array(64)), /./);
-throwsMsg(() => h5.read("/nope"), /./);
-throwsMsg(() => h5.list("/grid"), /not a group/);
-throwsMsg(() => h5.readHyperslab("/grid", [0], [1]), /dimensions/);
-throwsMsg(() => h5.readHyperslab("/grid", [5, 0], [2, 1]), /exceeds/);
-throwsMsg(() => h5.readHyperslab("/grid", [-1, 0], [1, 1]), /non-negative integers/);
-throwsMsg(() => h5.readHyperslab("/grid", [0.5, 0], [1, 1]), /non-negative integers/);
+await fails(() => pkg.open(new Uint8Array(64)), /./);
+await checkErrors(h5);
 // Info for a dataset with an unlimited dimension (netCDF "time").
 const nc = pkg.open(new Uint8Array(readFileSync(join(fixDir, "fixture.nc"))));
 eq(nc.info("/time").maxshape, [null], "unlimited dimension is null");
-// Big integers stay exact.
-eq(h5.read("/u64").data[0], 18446744073709551615n, "u64 max");
 eq(typeof pkg.version(), "string", "version");
 
 // Viewer helpers.
@@ -137,7 +157,486 @@ eq(lib.toRows(pairs.data, 2, 1, lib.perElement([2])), [["[2, 3]"], ["[4, 5]"]],
 eq(lib.formatValue(0.1 + 0.2), "0.3", "float formatting");
 eq(lib.formatValue(2n ** 64n - 1n), "18446744073709551615", "bigint formatting");
 eq(lib.formatValue("x"), '"x"', "string formatting");
+eq(lib.formatBytes(0), "0 B", "bytes");
+eq(lib.formatBytes(1536), "1.5 KiB", "KiB");
+eq(lib.formatBytes(200 * 1024 * 1024), "200 MiB", "MiB");
+eq(lib.formatStats({ lazy: true, requests: 3, bytesFetched: 2 << 20, size: 200 << 20 }),
+  "3 requests, 2 MiB of 200 MiB fetched (1.0%)", "stats line");
+eq(lib.formatStats({ lazy: false, requests: 1, bytesFetched: 1024, size: 1024 }),
+  "downloaded whole (1 KiB): the server does not support range requests", "stats line, no ranges");
 h5.free();
 nc.free();
-
 console.log(`wasm package: ${checks} checks passed`);
+
+if (base) await remoteTests();
+
+async function serverStats() {
+  return (await fetch(`${base}/__stats`)).json();
+}
+
+async function remoteTests() {
+  const before = checks;
+  await fetch(`${base}/__reset`);
+
+  // The same checks over HTTP range requests, at the default block size
+  // and at 512-byte blocks with a 4 KiB cache (almost every structure read
+  // a miss, evictions between calls).
+  for (const opts of [undefined, { blockSize: 512, cacheSize: 4096 }]) {
+    for (const [name, exp] of Object.entries(expected)) {
+      const f = await pkg.openUrl(`${base}/fix/${name}`, opts);
+      await checkFile(`${name} (openUrl ${JSON.stringify(opts ?? {})})`, exp, f);
+      const st = f.stats();
+      eq(st.lazy, true, "read by ranges");
+      eq(st.size, statSync(join(fixDir, name)).size, "size");
+      f.free();
+    }
+  }
+  const remote = await pkg.openUrl(`${base}/fix/fixture.h5`);
+  await checkErrors(remote);
+  eq((await (await pkg.openUrl(`${base}/fix/fixture.nc`)).info("/time")).maxshape, [null], "remote unlimited");
+
+  // What the page counts is what the server served.
+  await fetch(`${base}/__reset`);
+  const counted = await pkg.openUrl(`${base}/fix/fixture.nc`, { blockSize: 1024 });
+  await counted.read("/temp");
+  const server = await serverStats();
+  eq(counted.stats().requests, server.requests, "requests counted");
+  eq(counted.stats().bytesFetched, server.bytes, "bytes counted");
+
+  // A custom fetch is used for every request, with the caller's headers,
+  // given in any form fetch takes (a caller's Range is not sent).
+  for (const headers of [{ "X-Test": "1" }, new Headers({ "X-Test": "1", Range: "bytes=0-0" }), [["X-Test", "1"]]]) {
+    let calls = 0;
+    const seen = new Set();
+    const viaCustom = await pkg.openUrl(`${base}/fix/fixture.h5`, {
+      blockSize: 4096,
+      headers,
+      fetch: (url, init) => {
+        calls++;
+        const h = new Headers(init.headers);
+        seen.add(`${h.get("X-Test")} ${h.get("Range").startsWith("bytes=0-0") ? "caller's range" : "ours"}`);
+        return fetch(url, init);
+      },
+    });
+    const kind = headers.constructor.name;
+    eq((await viaCustom.read("/sensors/temp")).data[0], 21.5, `custom fetch values (${kind})`);
+    eq(calls, viaCustom.stats().requests, `custom fetch calls (${kind})`);
+    eq([...seen], ["1 ours"], `headers passed (${kind})`);
+  }
+
+  // parallel must be a positive integer.
+  for (const parallel of [0, -1, 1.5, "4", NaN, 5000]) {
+    await fails(() => pkg.openUrl(`${base}/fix/fixture.h5`, { parallel }), /parallel/, `parallel ${String(parallel)}`);
+  }
+
+  // When one range request fails, the others in flight are aborted and no
+  // more start (fetchRanges, called directly: remote.js is in the package).
+  {
+    const snippets = join(pkgDir, "snippets");
+    const dir = readdirSync(snippets).find((d) => existsSync(join(snippets, d, "js", "remote.js")));
+    const remote = await import(pathToFileURL(join(snippets, dir, "js", "remote.js")));
+    let started = 0;
+    const aborted = [];
+    const slowFetch = (url, init) => {
+      const k = started++;
+      if (k === 2) return Promise.resolve(new Response(null, { status: 500 }));
+      return new Promise((resolve, reject) => {
+        const t = setTimeout(() => resolve(new Response(new Uint8Array(10), { status: 206 })), 200);
+        init.signal?.addEventListener("abort", () => {
+          clearTimeout(t);
+          aborted.push(k);
+          reject(new DOMException("aborted", "AbortError"));
+        });
+      });
+    };
+    const ranges = Array.from({ length: 20 }, (_, i) => [i * 10, i * 10 + 10]).flat();
+    await fails(() => remote.fetchRanges("http://x.invalid/f.h5", ranges, { fetch: slowFetch, parallel: 3 }, null, 200),
+      /HTTP 500/, "a failed range request is the error");
+    await new Promise((r) => setTimeout(r, 300));
+    eq(started, 3, "no request starts after a failure");
+    eq(aborted.sort(), [0, 1], "requests in flight are aborted");
+    await fails(() => remote.fetchRanges("http://x.invalid/f.h5", ranges, { fetch: slowFetch, parallel: "x" }, null, 200),
+      /parallel must be a positive integer/, "fetchRanges checks parallel");
+  }
+
+  // Calls in flight at once share the cache (a 1 KiB budget: nothing is
+  // evicted while any of them runs) and each gets its own answer.
+  const both = await pkg.openUrl(`${base}/fix/fixture.h5`, { blockSize: 512, cacheSize: 1024 });
+  const [g, t, l, a] = await Promise.all([
+    both.read("/grid"), both.read("/sensors/temp"), both.list("/sensors"), both.attrs("/"),
+  ]);
+  const exp = expected["fixture.h5"];
+  eq(Array.from(g.data), exp.datasets["/grid"].values, "concurrent /grid");
+  eq(Array.from(t.data), exp.datasets["/sensors/temp"].values, "concurrent /sensors/temp");
+  eq(l.map((c) => c.name).sort(), [...exp.lists["/sensors"].groups, ...exp.lists["/sensors"].datasets].sort(), "concurrent list");
+  eq(a.length > 0, true, "concurrent attrs");
+  assert.ok(both.stats().cachedBytes <= 1024, "trimmed to the budget when idle");
+
+  // Listing and reading small things of a large file fetches a few blocks,
+  // not the file.
+  if (existsSync(join(fixDir, "big.json"))) {
+    const big = JSON.parse(readFileSync(join(fixDir, "big.json"), "utf8"));
+    await fetch(`${base}/__reset`);
+    const f = await pkg.openUrl(`${base}/fix/big.h5`);
+    const list = await f.list("/");
+    eq(list.filter((c) => c.kind === "group").map((c) => c.name), big.list.groups, "big: groups");
+    eq(list.filter((c) => c.kind === "dataset").map((c) => c.name).sort(), big.list.datasets, "big: datasets");
+    eq(Array.from((await f.read("/small")).data), big.small, "big: /small");
+    eq(Array.from((await f.read("/meta/ids")).data, String), big.ids, "big: /meta/ids");
+    eq((await f.info("/big")).shape, big.big_shape, "big: shape");
+    eq((await f.attrs("/meta"))[0].value, "m", "big: group attribute");
+    const win = await f.readHyperslab("/big", [big.window.start], [big.window.count]);
+    eq(Array.from(win.data), big.window.values, "big: window");
+    const st = f.stats();
+    const server = await serverStats();
+    eq(st.requests, server.requests, "big: requests counted");
+    eq(st.bytesFetched, server.bytes, "big: bytes counted");
+    console.log(`big.h5 (${big.size} bytes): listed, 2 small reads, attributes, info and a window in ` +
+      `${server.requests} requests, ${server.bytes} bytes (${(100 * server.bytes / big.size).toFixed(2)}%)`);
+    assert.ok(server.requests <= 8, `big: ${server.requests} requests`);
+    assert.ok(server.bytes * 20 < big.size, `big: ${server.bytes} bytes fetched`);
+    checks += 2;
+  }
+
+  // A cross-origin server that does not expose Content-Range, ETag or
+  // Last-Modified (serve.py's /noexpose/): the length comes from a HEAD
+  // request and answers are checked by their length alone. Every fixture
+  // check, calls in flight at once, and what the page counts.
+  for (const opts of [undefined, { blockSize: 512, cacheSize: 1024 }]) {
+    for (const [name, exp] of Object.entries(expected)) {
+      await fetch(`${base}/__reset`);
+      const f = await pkg.openUrl(`${base}/noexpose/fix/${name}`, opts);
+      await checkFile(`${name} (no exposed headers, ${JSON.stringify(opts ?? {})})`, exp, f);
+      const st = f.stats();
+      eq(st.lazy, true, "no exposed headers: read by ranges");
+      eq(st.size, statSync(join(fixDir, name)).size, "no exposed headers: size from HEAD");
+      const server = await serverStats();
+      eq(server.log.filter((l) => l[4] === "HEAD").length, 1, "no exposed headers: one HEAD");
+      eq(st.requests, server.requests, "no exposed headers: requests counted");
+      eq(st.bytesFetched, server.bytes, "no exposed headers: bytes counted");
+      f.free();
+    }
+  }
+  {
+    const f = await pkg.openUrl(`${base}/noexpose/fix/fixture.h5`, { blockSize: 512, cacheSize: 0, parallel: 3 });
+    const exp = expected["fixture.h5"];
+    const paths = ["/grid", "/sensors/temp", "/vlen_str", "/cube"];
+    const got = await Promise.all([...paths, ...paths].map((p) => f.read(p)));
+    got.forEach((r, i) => eq(values(exp.datasets[paths[i % 4]].kind, r.data), exp.datasets[paths[i % 4]].values,
+      `no exposed headers: concurrent ${paths[i % 4]}`));
+    await checkErrors(f);
+  }
+  // Without a validator a changed file cannot be told apart; a short
+  // answer still can.
+  await fails(async () => {
+    const f = await pkg.openUrl(`${base}/noexpose/fix/fixture.h5`, {
+      blockSize: 512,
+      fetch: async (url, init) => {
+        const r = await fetch(url, init);
+        if (init.method === "HEAD" || init.headers.Range === "bytes=0-511") return r;
+        return new Response((await r.arrayBuffer()).slice(1), { status: 206 });
+      },
+    });
+    await f.read("/grid");
+  }, /got \d+/, "no exposed headers: short answer");
+  // No HEAD length either: a clear error.
+  await fails(() => pkg.openUrl(`${base}/noexpose/fix/fixture.h5`, {
+    fetch: async (url, init) => (init.method === "HEAD" ? new Response(null, { status: 405 }) : fetch(url, init)),
+  }), /cannot learn the file's size/, "no exposed headers, no HEAD");
+
+  // A server without range support: downloaded whole (the default), or
+  // refused.
+  const whole = await pkg.openUrl(`${base}/norange/fix/fixture.h5`);
+  await checkFile("fixture.h5 (no range support)", expected["fixture.h5"], whole);
+  eq(whole.stats().lazy, false, "downloaded whole");
+  eq(whole.stats().requests, 1, "one request");
+  await fails(() => pkg.openUrl(`${base}/norange/fix/fixture.h5`, { fallback: "error" }),
+    /does not support HTTP range requests/, "fallback: error");
+  await fails(() => pkg.openUrl(`${base}/norange/fix/fixture.h5`, { maxDownload: 1000 }),
+    /more than maxDownload/, "maxDownload");
+  // Without a Content-Length the body is streamed, and stopped at the limit.
+  const undeclared = async (url, init) => {
+    const r = await fetch(url, init);
+    return new Response(r.body, { status: r.status });
+  };
+  await fails(() => pkg.openUrl(`${base}/norange/fix/fixture.h5`, { maxDownload: 1000, fetch: undeclared }),
+    /more than maxDownload/, "maxDownload, streamed");
+  const streamed = await pkg.openUrl(`${base}/norange/fix/fixture.h5`, { fetch: undeclared });
+  eq(Array.from((await streamed.read("/sensors/temp")).data), [21.5, 22, 22.25], "streamed download");
+
+  // Errors: HTTP status, not HDF5, bad options, a file that changes, a
+  // server that answers with the wrong bytes.
+  await fails(() => pkg.openUrl(`${base}/fix/missing.h5`), /HTTP 404/, "404");
+  await fails(() => pkg.openUrl(`${base}/fix/expected.json`), /./, "not HDF5");
+  await fails(() => pkg.openUrl(`${base}/fix/fixture.h5`, { blockSize: 100 }), /blockSize/, "blockSize");
+  const tamper = (edit) => async (url, init) => {
+    const r = await fetch(url, init);
+    return init.headers.Range === "bytes=0-511" ? r : edit(r);
+  };
+  const withHeaders = async (r, headers) => {
+    const h = new Headers(r.headers);
+    for (const [k, v] of Object.entries(headers)) h.set(k, v);
+    return new Response(await r.arrayBuffer(), { status: r.status, headers: h });
+  };
+  await fails(async () => {
+    const f = await pkg.openUrl(`${base}/fix/fixture.h5`, { blockSize: 512, fetch: tamper((r) => withHeaders(r, { ETag: '"other"' })) });
+    await f.read("/grid");
+  }, /changed on the server/, "changed file");
+  await fails(async () => {
+    const f = await pkg.openUrl(`${base}/fix/fixture.h5`, {
+      blockSize: 512,
+      fetch: tamper(async (r) => new Response((await r.arrayBuffer()).slice(1), { status: 206 })),
+    });
+    await f.read("/grid");
+  }, /got \d+/, "short answer");
+  await fails(async () => {
+    const f = await pkg.openUrl(`${base}/fix/fixture.h5`, {
+      blockSize: 512,
+      fetch: tamper(async (r) => new Response(await r.arrayBuffer(), { status: 200 })),
+    });
+    await f.read("/grid");
+  }, /stopped honouring range requests/, "200 mid-file");
+  await fails(async () => {
+    const f = await pkg.openUrl(`${base}/fix/fixture.h5`, {
+      blockSize: 512,
+      fetch: tamper(async (r) => withHeaders(r, { "Content-Range": "bytes 0-511/25752" })),
+    });
+    await f.read("/grid");
+  }, /the server sent 0-511/, "wrong range");
+  await fails(async () => {
+    const f = await pkg.openUrl(`${base}/fix/fixture.h5`, {
+      blockSize: 512,
+      fetch: tamper(async (r) => withHeaders(r, { "Content-Range": "bytes */25752" })),
+    });
+    await f.read("/grid");
+  }, /unusable Content-Range/, "unusable Content-Range");
+
+  await limitTests();
+  await floodTests();
+
+  // Corpus files: what the viewer can show of each is the same read by
+  // ranges as in memory (an error wherever it gives one).
+  const corpus = process.env.CLAWHDF5_WASM_CORPUS;
+  if (corpus) await corpusTests(corpus);
+  console.log(`openUrl: ${checks - before} checks passed`);
+}
+
+// A fetch that serves `buf` as a file of `total` bytes (zeros past the end
+// of `buf`, and `extra` = [[offset, bytes], ...] laid over them), counting
+// its calls. `hide` leaves out Content-Range and the validators, as a
+// cross-origin server that exposes neither does; HEAD then gives the length.
+function mockFetch(buf, { total = buf.length, extra = [], hide = false } = {}) {
+  const f = async (url, init) => {
+    f.calls++;
+    if (init.method === "HEAD") {
+      return new Response(null, { status: 200, headers: { "Content-Length": String(total) } });
+    }
+    const m = /^bytes=(\d+)-(\d+)$/.exec(new Headers(init.headers).get("Range"));
+    const a = Number(m[1]);
+    const b = Math.min(Number(m[2]) + 1, total);
+    const out = new Uint8Array(b - a);
+    for (const [at, bytes] of [[0, buf], ...extra]) {
+      const from = Math.max(a, at);
+      const to = Math.min(b, at + bytes.length);
+      if (from < to) out.set(bytes.subarray(from - at, to - at), from - a);
+    }
+    const headers = { "Content-Length": String(b - a) };
+    if (!hide) headers["Content-Range"] = `bytes ${a}-${b - 1}/${total}`;
+    return new Response(out, { status: 206, headers });
+  };
+  f.calls = 0;
+  return f;
+}
+
+// Sizes a hostile server or a large dataset can name are errors, never an
+// allocation that aborts the module (which would take every open file on
+// the page with it); see write_limits in make_fixture.py.
+async function limitTests() {
+  // Read whole, /huge_u8 (2^28 + 1024 bytes) would take over 2 GiB while
+  // decoding: refused before its chunks are fetched; a window reads.
+  const n = 2 ** 28 + 1024;
+  const limits = readFileSync(join(fixDir, "limits.h5"));
+  const local = pkg.open(new Uint8Array(limits));
+  await fails(() => local.read("/huge_u8"), /readHyperslab/, "huge read, in memory");
+  const remote = await pkg.openUrl(`${base}/fix/limits.h5`);
+  const before = remote.stats().requests;
+  await fails(() => remote.read("/huge_u8"), /readHyperslab/, "huge read, openUrl");
+  eq(remote.stats().requests, before, "huge read fetched nothing");
+  eq(Array.from((await remote.readHyperslab("/huge_u8", [n - 4], [4])).data), [0, 0, 0, 7], "huge window");
+  eq(Array.from(local.readHyperslab("/huge_u8", [n - 4], [4]).data), [0, 0, 0, 7], "huge window, in memory");
+  local.free();
+
+  // A server claiming 3 GiB, a heap collection claiming 2 GiB + 4 KiB: an
+  // error after a few requests (it used to fetch 2 GiB, then abort).
+  const hostile = mockFetch(new Uint8Array(readFileSync(join(fixDir, "hostile_vl.h5"))), { total: 3 * 2 ** 30 });
+  const h = await pkg.openUrl("http://hostile.invalid/h.h5", { fetch: hostile });
+  await fails(() => h.read("/a"), /maxFetch/, "hostile collection size");
+  assert.ok(hostile.calls <= 4, `hostile: ${hostile.calls} requests`);
+  // The module survived: files open and read.
+  eq((await (await pkg.openUrl(`${base}/fix/fixture.h5`)).read("/sensors/temp")).data[0], 21.5, "alive after hostile");
+
+  // A call that would fetch more than maxFetch fails before fetching it.
+  await fails(async () => {
+    const f = await pkg.openUrl(`${base}/fix/fixture.h5`, { blockSize: 512, maxFetch: 1024 });
+    await f.read("/grid");
+  }, /maxFetch/, "maxFetch");
+  await fails(() => pkg.openUrl(`${base}/fix/fixture.h5`, { maxFetch: 2 ** 31 }), /maxFetch/, "maxFetch range");
+  await fails(() => pkg.openUrl(`${base}/fix/fixture.h5`, { maxDownload: 2 ** 31 }), /maxDownload/, "maxDownload range");
+
+  // Offsets past 2 GiB work on wasm32: far.h5's data sits at 3 GiB.
+  const far = JSON.parse(readFileSync(join(fixDir, "far.json"), "utf8"));
+  const farBytes = new Uint8Array(readFileSync(join(fixDir, "far.h5")));
+  const farData = farBytes.subarray(far.data_at, far.data_at + far.nbytes);
+  const ff = await pkg.openUrl("http://far.invalid/far.h5", {
+    fetch: mockFetch(farBytes, { total: far.length, extra: [[far.far_at, farData]] }),
+  });
+  eq(Array.from((await ff.read("/x")).data), far.values, "data past 2 GiB");
+  eq(ff.stats().size, far.length, "size past 2 GiB");
+
+  // wasm32's reader holds file offsets in 32 bits: a file of 4 GiB or more
+  // is refused at open, with the limit in the message (past 2^53 - 1 bytes
+  // a JavaScript number is not even exact).
+  for (const total of [2 ** 32, 2 ** 40, 2 ** 53 + 2]) {
+    await fails(() => pkg.openUrl("http://huge.invalid/x.h5", { fetch: mockFetch(farBytes, { total }) }),
+      total > 2 ** 53 ? /2\^53 - 1/ : /4 GiB/, `length ${total}`);
+  }
+}
+
+// A body of `total` bytes in 64 KiB pieces, made as they are read; `pulled()`
+// tells how many were.
+function flood(total) {
+  let pulled = 0;
+  const body = new ReadableStream({
+    pull(c) {
+      if (pulled >= total) return c.close();
+      const n = Math.min(65536, total - pulled);
+      pulled += n;
+      c.enqueue(new Uint8Array(n));
+    },
+  }, { highWaterMark: 0 });
+  return { body, pulled: () => pulled };
+}
+
+// A 206 whose body runs past the range asked for is cut off as it arrives:
+// the page never reads (or holds) more than it asked for, whatever the
+// server sends. 64 MiB stands for "gigabytes".
+async function floodTests() {
+  const FLOOD = 64 << 20;
+  // The probe: asked for the first 1 MiB.
+  let f = flood(FLOOD);
+  await fails(() => pkg.openUrl("http://flood.invalid/x.h5", {
+    fetch: async () => new Response(f.body, { status: 206, headers: { "Content-Range": "bytes 0-1048575/2000000" } }),
+  }), /the server sent more/, "flooded probe");
+  assert.ok(f.pulled() <= (1 << 20) + 65536, `probe: read ${f.pulled()} bytes`);
+  // A range request after a correct probe.
+  f = flood(FLOOD);
+  const file = await pkg.openUrl(`${base}/fix/fixture.h5`, {
+    blockSize: 512,
+    fetch: async (url, init) => {
+      const range = new Headers(init.headers).get("Range");
+      if (range === "bytes=0-511") return fetch(url, init);
+      const m = /^bytes=(\d+)-(\d+)$/.exec(range);
+      return new Response(f.body, { status: 206, headers: { "Content-Range": `bytes ${m[1]}-${m[2]}/25752` } });
+    },
+  }).catch((e) => e);
+  // The open itself may need a second range: flooded either way.
+  const read = file instanceof Error ? Promise.reject(file) : file.read("/grid");
+  await fails(() => read, /the server sent more/, "flooded range");
+  assert.ok(f.pulled() <= 8 * 512 + 65536, `range: read ${f.pulled()} bytes`);
+  // A declared Content-Length past the range is refused before reading.
+  f = flood(FLOOD);
+  await fails(() => pkg.openUrl("http://flood.invalid/x.h5", {
+    fetch: async () => new Response(f.body, {
+      status: 206, headers: { "Content-Range": "bytes 0-1048575/2000000", "Content-Length": String(FLOOD) },
+    }),
+  }), /the server sent more/, "declared too long");
+  assert.ok(f.pulled() <= 65536, `declared: read ${f.pulled()} bytes`);
+}
+
+function hdf5Files(dir, out) {
+  for (const e of readdirSync(dir, { withFileTypes: true })) {
+    const p = join(dir, e.name);
+    let st;
+    try {
+      st = statSync(p);
+    } catch {
+      continue;
+    }
+    if (st.isDirectory()) hdf5Files(p, out);
+    else if (st.size <= 16 << 20) {
+      const b = readFileSync(p);
+      const sig = [0x89, 0x48, 0x44, 0x46, 0x0d, 0x0a, 0x1a, 0x0a];
+      for (let at = 0; at + 8 <= b.length; at = at === 0 ? 512 : at * 2) {
+        if (sig.every((x, i) => b[at + i] === x)) { out.push(p); break; }
+      }
+    }
+  }
+  return out;
+}
+
+function show(v) {
+  return JSON.stringify(v, (_, x) => {
+    if (typeof x === "bigint") return `${x}n`;
+    if (ArrayBuffer.isView(x)) return Array.from(x, (y) => (typeof y === "bigint" ? `${y}n` : Number.isNaN(y) ? "NaN" : y));
+    return x;
+  });
+}
+
+async function transcript(f) {
+  const out = [];
+  const call = async (what, fn) => {
+    try {
+      out.push(`${what}: ${show(await fn())}`);
+    } catch {
+      out.push(`${what}: Err`);
+    }
+  };
+  const todo = [["/", 0]];
+  while (todo.length && out.length < 3000) {
+    const [path, depth] = todo.pop();
+    let kind = null;
+    await call(`${path} kind`, async () => (kind = await f.kind(path)));
+    await call(`${path} attrs`, () => f.attrs(path));
+    if (kind === "group") {
+      let list = [];
+      await call(`${path} list`, async () => (list = await f.list(path)));
+      if (depth < 12) for (const c of list.reverse()) todo.push([lib.joinPath(path, c.name), depth + 1]);
+    } else if (kind === "dataset") {
+      let info = null;
+      await call(`${path} info`, async () => (info = await f.info(path)));
+      if (info && [...info.shape, ...info.elementShape].reduce((a, b) => a * b, 1) <= 1 << 20) {
+        await call(`${path} read`, () => f.read(path));
+      }
+    }
+  }
+  return out;
+}
+
+async function corpusTests(corpus) {
+  const files = hdf5Files(corpus, []).sort();
+  let opened = 0;
+  let bytes = 0;
+  let size = 0;
+  for (const p of files) {
+    const rel = relative(corpus, p).split("/").map(encodeURIComponent).join("/");
+    let local;
+    try {
+      local = pkg.open(new Uint8Array(readFileSync(p)));
+    } catch {
+      await fails(() => pkg.openUrl(`${base}/corpus/${rel}`, { blockSize: 65536 }), /./, `${rel}: opens in neither`);
+      continue;
+    }
+    const remote = await pkg.openUrl(`${base}/corpus/${rel}`, { blockSize: 65536 });
+    const want = await transcript(local);
+    const got = await transcript(remote);
+    // Errors are compared as errors: a malformed file can fail at another
+    // check, with another message, when read by ranges.
+    eq(got, want, `${rel}: transcript`);
+    opened++;
+    bytes += remote.stats().bytesFetched;
+    size += remote.stats().size;
+    local.free();
+    remote.free();
+  }
+  console.log(`corpus: ${opened} of ${files.length} files agree over HTTP (${bytes} of ${size} bytes fetched)`);
+}
diff --git a/examples/wasm-viewer/viewer-lib.js b/examples/wasm-viewer/viewer-lib.js
index 72c5bf3..fece1ab 100644
--- a/examples/wasm-viewer/viewer-lib.js
+++ b/examples/wasm-viewer/viewer-lib.js
@@ -75,3 +75,24 @@ export function toRows(data, rows, cols, per = 1) {
 export function perElement(elementShape) {
   return elementShape.reduce((a, b) => a * b, 1);
 }
+
+/** A byte count for people: `1.5 KiB`, `200 MiB`. */
+export function formatBytes(n) {
+  const units = ["B", "KiB", "MiB", "GiB", "TiB"];
+  let i = 0;
+  let v = n;
+  while (v >= 1024 && i < units.length - 1) {
+    v /= 1024;
+    i++;
+  }
+  const s = i === 0 ? String(v) : v < 10 ? String(Number(v.toFixed(1))) : String(Math.round(v));
+  return `${s} ${units[i]}`;
+}
+
+/** What reading a remote file has cost, from `RemoteFile.stats()`. */
+export function formatStats({ lazy, requests, bytesFetched, size }) {
+  if (!lazy) return `downloaded whole (${formatBytes(size)}): the server does not support range requests`;
+  const pct = size > 0 ? (100 * bytesFetched) / size : 0;
+  return `${requests} request${requests === 1 ? "" : "s"}, ${formatBytes(bytesFetched)} of ` +
+    `${formatBytes(size)} fetched (${pct.toFixed(1)}%)`;
+}