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..7689ff1 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: $CHROME, else chromium/google-chrome on PATH, else a Playwright
 # download under ~/.cache/ms-playwright. Exit 3 when none is found.
@@ -31,63 +34,76 @@ if [ -z "$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 $H5 "/table" 'class="error"' 'reading compound{x: f64, n: i32} datasets is not supported'
+# A server without range support: the file is downloaded whole, and says so.
+expect norange/$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
