diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
deleted file mode 100644
index 68afe9b..0000000
--- a/.github/workflows/build.yml
+++ /dev/null
@@ -1,15 +0,0 @@
-name: Build spec
-
-on: [pull_request, push]
-
-jobs:
-  build:
-    runs-on: ubuntu-latest
-
-    steps:
-    - uses: actions/checkout@v4
-    - uses: ljharb/actions/node/install@main
-      name: 'nvm install lts/* && npm install'
-      with:
-        node-version: lts/*
-    - run: npm run build
diff --git a/.github/workflows/deploy.yml b/.github/workflows/publish-main.yml
similarity index 68%
rename from .github/workflows/deploy.yml
rename to .github/workflows/publish-main.yml
index 90d6c76..bf9776f 100644
--- a/.github/workflows/deploy.yml
+++ b/.github/workflows/publish-main.yml
@@ -1,4 +1,4 @@
-name: Deploy gh-pages
+name: Publish main
 
 on:
   push:
@@ -6,9 +6,8 @@ on:
       - main
 
 jobs:
-  deploy:
+  publish:
     runs-on: ubuntu-latest
-
     steps:
     - uses: actions/checkout@v4
     - uses: ljharb/actions/node/install@main
@@ -16,8 +15,10 @@ jobs:
       with:
         node-version: lts/*
     - run: npm run build
-    - uses: JamesIves/github-pages-deploy-action@v4
+    - name: Publish to gh-pages
+      uses: JamesIves/github-pages-deploy-action@v4
       with:
         branch: gh-pages
         folder: build
-        clean: true
+        clean-exclude: |
+          pr
diff --git a/.github/workflows/publish-pr.yml b/.github/workflows/publish-pr.yml
new file mode 100644
index 0000000..14399bc
--- /dev/null
+++ b/.github/workflows/publish-pr.yml
@@ -0,0 +1,110 @@
+name: Publish PR
+run-name: ${{ github.event.workflow_run.display_title }}
+
+on:
+  workflow_run:
+    workflows: ['Render PR']
+    types: [completed]
+
+env:
+  # Must match ./render-pr.yml
+  ARTIFACT_NAME: ${{ vars.ARTIFACT_NAME || 'result' }}
+
+jobs:
+  publish:
+    runs-on: ubuntu-latest
+    if: >
+      ${{
+        !github.event.repository.fork
+        && github.event.workflow_run.event == 'pull_request'
+        && github.event.workflow_run.conclusion == 'success'
+      }}
+    steps:
+    - uses: actions/checkout@v4
+    - uses: ljharb/actions/node/install@main
+      name: 'nvm install lts/* && npm install'
+      with:
+        node-version: lts/*
+    - name: Print event info
+      uses: actions/github-script@v7
+      with:
+        script: 'console.log(${{ toJson(github.event) }});'
+    - name: Download zipball
+      uses: actions/github-script@v7
+      with:
+        script: |
+          const { owner, repo } = context.repo;
+          const run_id = ${{ github.event.workflow_run.id }};
+          const name = process.env.ARTIFACT_NAME;
+          const listArtifactsQuery = { owner, repo, run_id, name };
+          const listArtifactsResponse = await github.rest.actions.listWorkflowRunArtifacts(listArtifactsQuery);
+          const { total_count, artifacts } = listArtifactsResponse.data;
+          if (total_count !== 1) {
+            const summary = artifacts?.map(({ name, size_in_bytes, url }) => ({ name, size_in_bytes, url }));
+            const detail = JSON.stringify(summary ?? []);
+            throw new RangeError(`Expected 1 ${name} artifact, got ${total_count} ${detail}`);
+          }
+          const artifact_id = artifacts[0].id;
+          console.log(`downloading artifact ${artifact_id}`);
+          const downloadResponse = await github.rest.actions.downloadArtifact({
+            owner,
+            repo,
+            artifact_id,
+            archive_format: 'zip',
+          });
+          const fs = require('fs');
+          fs.writeFileSync('${{ github.workspace }}/result.zip', Buffer.from(downloadResponse.data));
+    - name: Provide result directory
+      run: rm -rf result && mkdir -p result
+    - run: unzip -o result.zip -d result
+    - run: ls result
+    - name: Extract PR data
+      run: |
+        cd result
+        awk -v ok=1 '
+          NR == 1 && match($0, /^[1-9][0-9]* [0-9a-fA-F]{7,}$/) {
+            print "PR=" $1;
+            print "COMMIT=" $2;
+            next;
+          }
+          { ok = 0; }
+          END { exit !ok; }
+        ' pr-data.txt >> "$GITHUB_ENV"
+        rm pr-data.txt
+    - name: Insert preview warning
+      run: |
+        tmp="$(mktemp -u XXXXXXXX.json)"
+        export REPO_URL="https://github.com/$GITHUB_REPOSITORY"
+        jq -n '
+          def repo_link($args): $args as [$path, $contents]
+            | (env.REPO_URL + ($path // "")) as $url
+            | "<a href=\"\($url | @html)\">\($contents // $url)</a>";
+          {
+            SUMMARY: "PR #\(env.PR)",
+            REPO_LINK: repo_link([]),
+            PR_LINK: repo_link(["/pull/" + env.PR, "PR #\(env.PR)"]),
+            COMMIT_LINK: ("commit " + repo_link(["/commit/" + env.COMMIT, "<code>\(env.COMMIT)</code>"])),
+          }
+        ' > "$tmp"
+        find result -name '*.html' -exec \
+          node scripts/insert_warning.mjs scripts/pr_preview_warning.html "$tmp" '{}' '+'
+    - name: Publish to gh-pages
+      uses: JamesIves/github-pages-deploy-action@v4
+      with:
+        branch: gh-pages
+        folder: result
+        target-folder: pr/${{ env.PR }}
+    - name: Determine gh-pages url
+      id: get-pages-url
+      run: |
+        gh_pages_url="https://$(printf '%s' "$GITHUB_REPOSITORY" \
+          | sed 's#/#.github.io/#; s#^tc39.github.io/#tc39.es/#')"
+        echo "url=$gh_pages_url" >> $GITHUB_OUTPUT
+    - name: Provide PR comment
+      uses: phulsechinmay/rewritable-pr-comment@v0.3.0
+      with:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        ISSUE_ID: ${{ env.PR }}
+        message: >
+          The rendered spec for this PR is available at
+          ${{ steps.get-pages-url.outputs.url }}/pr/${{ env.PR }}.
diff --git a/.github/workflows/render-pr.yml b/.github/workflows/render-pr.yml
new file mode 100644
index 0000000..fc7537e
--- /dev/null
+++ b/.github/workflows/render-pr.yml
@@ -0,0 +1,39 @@
+name: Render PR
+
+on: [pull_request]
+
+env:
+  # Must match ./publish-pr.yml
+  ARTIFACT_NAME: ${{ vars.ARTIFACT_NAME || 'result' }}
+
+jobs:
+  render:
+    runs-on: ubuntu-latest
+    if: ${{ github.event.pull_request }}
+    steps:
+    - uses: actions/checkout@v4
+    - name: '[node LTS] npm install'
+      uses: ljharb/actions/node/install@main
+      with:
+        node-version: lts/*
+    - run: npm run build
+    - name: Save PR data
+      env:
+        PR: ${{ github.event.number }}
+      run: echo "$PR $(git rev-parse --verify HEAD)" > build/pr-data.txt
+    - uses: actions/upload-artifact@v4
+      id: upload
+      if: ${{ !github.event.repository.fork }}
+      with:
+        name: ${{ env.ARTIFACT_NAME }}
+        path: build/
+    - name: Echo artifact ID
+      run: echo 'Artifact ID is ${{ steps.upload.outputs.artifact-id }}'
+    - name: Verify artifact discoverability
+      uses: actions/github-script@v7
+      with:
+        script: |
+          const { owner, repo } = context.repo;
+          const run_id = ${{ github.run_id }};
+          const listArtifactsResponse = await github.rest.actions.listWorkflowRunArtifacts({ owner, repo, run_id });
+          console.log(`artifacts for run id ${run_id}`, listArtifactsResponse?.data);
diff --git a/package.json b/package.json
index 438a1fa..e46c4c0 100644
--- a/package.json
+++ b/package.json
@@ -15,7 +15,10 @@
   "license": "MIT",
   "devDependencies": {
     "@tc39/ecma262-biblio": "^2.1.2816",
-    "ecmarkup": "^20.0.0"
+    "ecmarkup": "^20.0.0",
+    "jsdom": "^25.0.1",
+    "parse5-html-rewriting-stream": "^7.0.0",
+    "tmp": "^0.2.3"
   },
   "engines": {
     "node": ">= 18"
diff --git a/scripts/insert_warning.mjs b/scripts/insert_warning.mjs
new file mode 100644
index 0000000..f493d78
--- /dev/null
+++ b/scripts/insert_warning.mjs
@@ -0,0 +1,141 @@
+import fs from 'fs';
+import { readFile, rename } from 'fs/promises';
+import pathlib from 'path';
+import { pipeline } from 'stream/promises';
+import { parseArgs } from 'util';
+import { JSDOM } from 'jsdom';
+import { RewritingStream } from 'parse5-html-rewriting-stream';
+import tmp from 'tmp';
+
+const { positionals: cliArgs } = parseArgs({
+  allowPositionals: true,
+  options: {},
+});
+if (cliArgs.length < 3) {
+  const self = pathlib.relative(process.cwd(), process.argv[1]);
+  console.error(`Usage: node ${self} <template.html> <data.json> <file.html>...
+
+{{identifier}} substrings in template.html are replaced from data.json, then
+the result is inserted into each file.html.`);
+  process.exit(64);
+}
+
+const main = async ([templateFile, dataFile, ...files]) => {
+  // Substitute data into the template.
+  const [
+    template,
+    data,
+  ] = await Promise.all([
+    readFile(templateFile, 'utf8'),
+    readFile(dataFile, 'utf8').then(JSON.parse),
+  ])
+  const formatErrors = [];
+  const placeholderPatt = /[{][{](?:([\p{ID_Start}$_][\p{ID_Continue}$]*)[}][}]|.*?(?:[}][}]|(?=[{][{])|$))/gsu;
+  const resolved = template.replaceAll(placeholderPatt, (m, name, i) => {
+    if (!name) {
+      const trunc = m.replace(/([^\n]{29}(?!$)|[^\n]{,29}(?=\n)).*/s, '$1…');
+      formatErrors.push(new SyntaxError(`bad placeholder at index ${i}: ${trunc}`));
+    } else if (!Object.hasOwn(data, name)) {
+      formatErrors.push(new ReferenceError(`no data for ${m}`));
+    }
+    return data[name];
+  });
+  if (formatErrors.length > 0) {
+    throw new AggregateError(formatErrors);
+  }
+
+  // Parse the template into DOM nodes for appending to page head (metadata such
+  // as <style> elements) or prepending to page body (everything else).
+  const jsdomOpts = { contentType: 'text/html; charset=utf-8' };
+  const { document } = new JSDOM(resolved, jsdomOpts).window;
+  const headHTML = document.head.innerHTML;
+  const bodyHTML = document.body.innerHTML;
+
+  // Perform the insertions.
+  const work = files.map(async (file) => {
+    await null;
+    const { name: tmpName, fd, removeCallback } = tmp.fileSync({
+      tmpdir: pathlib.dirname(file),
+      prefix: pathlib.basename(file),
+      postfix: '.tmp',
+      detachDescriptor: true,
+    });
+    try {
+      // Make a pipeline: fileReader -> inserter -> finisher -> fileWriter
+      const fileReader = fs.createReadStream(file, 'utf8');
+      const fileWriter = fs.createWriteStream('', { fd, flush: true });
+
+      // Insert headHTML at the end of a possibly implied head, and bodyHTML at
+      // the beginning of a possibly implied body.
+      // https://html.spec.whatwg.org/multipage/parsing.html#parsing-main-inhtml
+      let mode = 'before html'; // | 'before head' | 'in head' | 'after head' | '$DONE'
+      const stayInHead = new Set([
+        'base',
+        'basefont',
+        'bgsound',
+        'head',
+        'link',
+        'meta',
+        'noframes',
+        'noscript',
+        'script',
+        'style',
+        'template',
+        'title',
+      ]);
+      const inserter = new RewritingStream();
+      const onEndTag = function (tag) {
+        if (tag.tagName === 'head') {
+          this.emitRaw(headHTML);
+          mode = 'after head';
+        }
+        this.emitEndTag(tag);
+      };
+      const onStartTag = function (tag) {
+        const echoTag = () => this.emitStartTag(tag);
+        if (mode === 'before html' && tag.tagName === 'html') {
+          mode = 'before head';
+        } else if (mode !== 'after head' && stayInHead.has(tag.tagName)) {
+          mode = 'in head';
+        } else {
+          if (mode !== 'after head') { this.emitRaw(headHTML); }
+          // Emit either `${bodyTag}${bodyHTML}` or `${bodyHTML}${otherTag}`.
+          const emits = [echoTag, () => this.emitRaw(bodyHTML)];
+          if (tag.tagName !== 'body') { emits.reverse(); }
+          for (const emit of emits) { emit(); }
+          mode = '$DONE';
+          this.off('endTag', onEndTag).off('startTag', onStartTag);
+          return;
+        }
+        echoTag();
+      };
+      inserter.on('endTag', onEndTag).on('startTag', onStartTag);
+
+      // Ensure headHTML/bodyHTML insertion before EOF.
+      const finisher = async function* (source) {
+        for await (const chunk of source) { yield chunk; }
+        if (mode === '$DONE') { return; }
+        if (mode !== 'after head') { yield headHTML; }
+        yield bodyHTML;
+      };
+
+      await pipeline(fileReader, inserter, finisher, fileWriter);
+
+      // Now that the temp file is complete, overwrite the source file.
+      await rename(tmpName, file);
+    } finally {
+      removeCallback();
+    }
+  });
+  const results = await Promise.allSettled(work);
+
+  const failures = results.filter((result) => result.status !== 'fulfilled');
+  if (failures.length > 0) {
+    throw new AggregateError(failures.map((r) => r.reason));
+  }
+};
+
+main(cliArgs).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..efc3553
--- /dev/null
+++ b/scripts/pr_preview_warning.html
@@ -0,0 +1,69 @@
+<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: 40%;
+    translate: 0 calc(-1 * min(50%, 40vh));
+    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;
+    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}} as {{COMMIT_LINK}}.
+  </p>
+  <p>
+    Do not reference it as authoritative in any way.
+    Instead, see the living specification at {{REPO_LINK}}.
+  </p>
+</details>
+<script>
+  {
+    const dismissWarnings = evt => {
+      if (evt.code !== 'Escape') return;
+      for (const el of document.querySelectorAll('details.annoying-warning[open]')) {
+        el.open = false;
+      }
+    };
+    document.addEventListener('keydown', dismissWarnings, { passive: true });
+  }
+</script>