diff --git a/.github/workflows/publish-pr.yml b/.github/workflows/publish-pr.yml
index edca4e0..0adb2b9 100644
--- a/.github/workflows/publish-pr.yml
+++ b/.github/workflows/publish-pr.yml
@@ -57,11 +57,31 @@ jobs:
       id: extract-pr-number
       run: |
         cd result
-        awk -vok=1 \
+        awk -v ok=1 \
           '{ print; if(!match($0, /^[1-9][0-9]*$/)) ok=0; } END { exit !(NR==1 && ok); }' \
           pr-number.txt
         echo "number=$(cat pr-number.txt)" >> $GITHUB_OUTPUT
         rm pr-number.txt
+    - name: Insert preview warning
+      env:
+        PR: ${{ steps.extract-pr-number.outputs.number }}
+      run: |
+        tmp="$(mktemp -u XXXXXXXX.json)"
+        repo_url="https://github.com/$GITHUB_REPOSITORY"
+        commit="$(git rev-parse --verify HEAD)"
+        jq -n --arg repo_url "$repo_url" --arg PR "$PR" --arg commit "$commit" '
+          def repo_link($args): $args as [$path, $contents]
+            | ($repo_url + ($path // "")) as $url
+            | "<a href=\"\($url | @html)\">\($contents // $url)</a>";
+          {
+            SUMMARY: "PR #\($PR)",
+            REPO_LINK: repo_link([]),
+            PR_LINK: repo_link(["/pull/" + $PR, "PR #\($PR)"]),
+            COMMIT_LINK: ("commit " + repo_link(["/commit/" + $commit, "<code>\($commit)</code>"])),
+          }
+        ' > "$tmp"
+        find result -name '*.html' -exec \
+          node scripts/insert_warning.mjs --strict scripts/pr_preview_warning.html "$tmp" '{}' '+'
     - name: Publish to gh-pages
       uses: JamesIves/github-pages-deploy-action@v4.3.3
       with:
diff --git a/package.json b/package.json
index abb4689..98ccf43 100644
--- a/package.json
+++ b/package.json
@@ -17,7 +17,8 @@
   "license": "MIT",
   "devDependencies": {
     "@tc39/ecma262-biblio": "^2.1.2775",
-    "ecmarkup": "^20.0.0"
+    "ecmarkup": "^20.0.0",
+    "jsdom": "^25.0.1"
   },
   "engines": {
     "node": ">= 12"
diff --git a/scripts/insert_warning.mjs b/scripts/insert_warning.mjs
new file mode 100644
index 0000000..6c6fc7c
--- /dev/null
+++ b/scripts/insert_warning.mjs
@@ -0,0 +1,73 @@
+import fs from 'node:fs';
+import pathlib from 'node:path';
+import { parseArgs } from 'node:util';
+import { JSDOM, VirtualConsole } from 'jsdom';
+
+const { positionals: cliArgs, values: cliOpts } = parseArgs({
+  allowPositionals: true,
+  options: {
+    strict: { type: 'boolean' },
+  },
+});
+if (cliArgs.length < 3) {
+  const self = pathlib.relative(process.cwd(), process.argv[1]);
+  console.error(`Usage: node ${self} [--strict] <template.html> <data.json> <file.html>...
+
+{{identifier}} substrings in template.html are replaced from data.json, then
+the result is inserted at the start of the body element in each file.html.`);
+  process.exit(64);
+}
+
+const main = async (args, options) => {
+  const [templateFile, dataFile, ...files] = args;
+  const { strict } = options;
+
+  // Evaluate the template and parse it into nodes for inserting.
+  // Everything will be prepended to body elements except metadata elements,
+  // which will be appended to head elements.
+  // https://html.spec.whatwg.org/multipage/dom.html#metadata-content-2
+  const metadataNames =
+    'base, link, meta, noscript, script, style, template, title'
+      .toUpperCase()
+      .split(', ');
+  const template = fs.readFileSync(templateFile, 'utf8');
+  const { default: data } =
+    await import(pathlib.resolve(dataFile), { with: { type: 'json' } });
+  const namePatt = /[{][{]([\p{ID_Start}$_][\p{ID_Continue}$]*)[}][}]/gu;
+  const resolved = template.replaceAll(namePatt, (_, name) => {
+    if (Object.hasOwn(data, name)) return data[name];
+    if (strict) throw Error(`no data for {{${name}}}`);
+    return '';
+  });
+  const headInserts = [], bodyInserts = [];
+  let insertDom = JSDOM.fragment(resolved);
+  for (const node of insertDom.childNodes) {
+    if (metadataNames.includes(node.nodeName)) headInserts.push(node);
+    else bodyInserts.push(node);
+  }
+
+  // Perform the insertions, suppressing JSDOM warnings from e.g. unsupported
+  // CSS features.
+  const virtualConsole = new VirtualConsole();
+  virtualConsole.on('error', () => {});
+  const jsdomOpts = { contentType: 'text/html; charset=utf-8', virtualConsole };
+  const getInserts =
+    files.length > 1 ? nodes => nodes.map(n => n.cloneNode(true)) : x => x;
+  const results = await Promise.allSettled(files.map(async file => {
+    let dom = await JSDOM.fromFile(file, jsdomOpts);
+    const { head, body } = dom.window.document;
+    if (headInserts.length > 0) head.append(...getInserts(headInserts));
+    if (bodyInserts.length > 0) body.prepend(...getInserts(bodyInserts));
+    fs.writeFileSync(file, dom.serialize(), 'utf8');
+  }));
+
+  const failures = results.flatMap(result =>
+    result.status === 'fulfilled' ? [] : [result.reason],
+  );
+  if (failures.length > 0) throw AggregateError(failures);
+};
+
+main(cliArgs, cliOpts).catch(err => {
+  console.error(err);
+  process.exit(1);
+});
diff --git a/scripts/pr_preview_warning.html b/scripts/pr_preview_warning.html
new file mode 100644
index 0000000..92b7c04
--- /dev/null
+++ b/scripts/pr_preview_warning.html
@@ -0,0 +1,58 @@
+<style>
+  details.annoying-warning {
+    position: fixed;
+    top: 0;
+    right: 0;
+    z-index: 10;
+    border: 2px solid white;
+    background-color: #920800;
+    background-image: linear-gradient(transparent 40%, rgba(255, 255, 255, 0.2));
+    color: white;
+    opacity: .95;
+  }
+
+  details.annoying-warning[open] {
+    top: 10%;
+    top: calc(5vw + 5vh);
+    left: 5%;
+    right: 5%;
+    margin: 0 auto;
+    max-width: 800px;
+    outline: solid 10000px rgba(255, 255, 255, 0.6);
+    border-radius: 3px;
+    border-width: 1px 1px 0 1px;
+    box-shadow: 0 0 0.5em rgba(0, 0, 0, 0.5);
+  }
+
+  details.annoying-warning > summary {
+    padding: 0.5ex 1ex;
+    font-size: 0.875em;
+    font-weight: bold;
+    letter-spacing: 0.02em;
+    text-align: center;
+    text-shadow: 0px 1px 2px rgba(0, 0, 0, 0.85);
+    cursor: default;
+  }
+
+  details.annoying-warning > p {
+    line-height: 1.4;
+    margin: 1em;
+    text-shadow: 0px 1px 1px rgba(0, 0, 0, 0.85);
+  }
+
+  details.annoying-warning a {
+    color: inherit;
+    text-decoration: underline;
+  }
+</style>
+
+<details class="annoying-warning">
+  <summary>{{SUMMARY}}</summary>
+  <p>
+    This document is a preview of merging {{PR_LINK}}, resulting in {{COMMIT_LINK}}.
+  </p>
+  <p>
+    Do not reference it as authoritative in any way.
+    Instead, see {{REPO_LINK}} for the living specification.
+  </p>
+</details>