diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1fc429d..7c44dbf 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -280,6 +280,32 @@
   infinity; batches are all or nothing. CLI: `create --float16`. See
   `BENCHMARKS.md`, "float16 embedding storage".
 
+### Browser (WebAssembly)
+- **New crate `clawhdf5-wasm`:** the reader compiled to
+  `wasm32-unknown-unknown` with a wasm-bindgen JavaScript API —
+  `open(bytes)`, `list`, `info`, `attrs`, `read`, `readHyperslab` — returning
+  typed arrays of the stored width (`BigInt64Array` for 64-bit integers),
+  string arrays for strings and enums, and a thrown `Error` for types with no
+  typed-array form (compound, reference, opaque, VL sequences) or filters the
+  build lacks (Zstd, SZIP). Read-only; the file is held in memory.
+- **`examples/wasm-viewer/`:** a drop-a-file HDF5/NetCDF-4 viewer page (tree,
+  type/shape/attributes, values paged as hyperslabs; `?file=&path=` opens a
+  URL). `build.sh` produces the package; `test/run.sh` checks it under Node
+  (251 checks against values h5py/libhdf5 read back from an h5py- and a
+  netCDF4-written file) and renders the page in headless Chromium. Size,
+  measured 2026-09-26 on tank (`gzip -9 -n`): 627,501 B of wasm, 191,639 B
+  gzipped, plus 21,826 B (4,487 B) of JS glue; h5wasm 0.10.3's embedded wasm
+  is 3,544,184 B (907,096 B) — full libhdf5, so not equal functionality. See
+  `examples/wasm-viewer/README.md`.
+- The facade's read path already built for `wasm32-unknown-unknown` (nothing
+  needed gating); `ci-test.sh` now builds it (`--no-default-features`) and
+  lints `clawhdf5-wasm` for that target, and CI installs the target. The Node
+  and browser tests run in `ci-test.sh` only where `node` and `wasm-bindgen`
+  exist (not the CI container); CI checks the same expectations natively
+  (`clawhdf5-wasm`'s `h5py_interop` test).
+- `Dataset::raw_datatype()` (facade) returns the full stored datatype, for
+  decoding `read_selection` bytes with `clawhdf5_format::data_read`.
+
 ### Build
 - **Pure-Rust default.** `clawhdf5-format`, `clawhdf5-filters` and the
   `clawhdf5` facade default to the `zlib-rs` deflate backend; `fast-deflate`
diff --git a/CLAUDE.md b/CLAUDE.md
index 4969c6a..c21af88 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -150,6 +150,14 @@ Cargo workspace with 17 crates under `crates/` (plus `libaec-sys`, an internal F
   `MemorySource` for this bookkeeping is inferred from the caller-supplied
   `source_channel` string (a heuristic, not an authenticated trust boundary).
 - 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.
 - Python and Node.js bindings for cross-language use
 - NetCDF-4 compatibility for scientific data interop
 
diff --git a/docs/known-issues.md b/docs/known-issues.md
index 4ea7518..ca4282b 100644
--- a/docs/known-issues.md
+++ b/docs/known-issues.md
@@ -422,6 +422,23 @@ the same agent-store interop test.
 **Fix:** an empty contiguous dataset gets the undefined address (all `0xff`),
 which is what libhdf5 itself writes.
 
+## `clawhdf5-wasm` (browser) limits
+
+**Status:** open (by design for now; added 2026-09-26).
+
+- 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.
+- 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`.
+- No Zstd or SZIP (both link C): such datasets fail with
+  `unsupported filter: 32015` / `: 4`. pcodec is not enabled either.
+- External links and virtual-dataset sources in other files cannot be
+  followed (no file system).
+- Variable-length string datasets are read by decoding `read_selection`'s
+  bytes with `clawhdf5_format::vl_data` in the wasm crate; `File` itself still
+  cannot (see the audit gaps above).
+
 ## The Node.js package (`packages/clawhdf5-node`) does not work
 
 **Status:** open (found 2026-09-25). Unpublished; not built or tested in CI.