diff --git a/craft/package.json b/craft/package.json
index 5789f0c..f8d4921 100644
--- a/craft/package.json
+++ b/craft/package.json
@@ -13,6 +13,7 @@
     "test:unit": "vitest run",
     "test:browser:bg": "node scripts/e2e-background.mjs",
     "test:browser:canvas": "node scripts/e2e-canvas-parity.mjs",
+    "test:browser:mobile": "node scripts/e2e-mobile-parity.mjs",
     "test:unit:watch": "vitest"
   },
   "dependencies": {
diff --git a/craft/scripts/e2e-mobile-parity-entry.ts b/craft/scripts/e2e-mobile-parity-entry.ts
new file mode 100644
index 0000000..3ab00e3
--- /dev/null
+++ b/craft/scripts/e2e-mobile-parity-entry.ts
@@ -0,0 +1,73 @@
+// Bundled by scripts/e2e-mobile-parity.mjs. Builds the documents the real-browser mobile test measures.
+// Uses only exports that exist before and after the mobile-parity fix so the SAME harness can record a baseline.
+import { exportBodyHtml, exportComposedPreviewHtml } from '../src/utils/html-export';
+
+const node = (type: string, props: Record<string, unknown>, nodes: string[] = [], extra: Record<string, unknown> = {}) => ({
+  type: { resolvedName: type }, isCanvas: type === 'Container', props, displayName: type, custom: {}, hidden: false, nodes, linkedNodes: {}, ...extra,
+});
+const LONG = 'Honest prices, straight answers and work that is finished when we say it will be, every time.';
+
+/** ColumnLayout with `n` Container columns each holding one TextBlock. */
+function addColumns(nodes: Record<string, any>, id: string, props: Record<string, unknown>, n: number) {
+  const linked: Record<string, string> = {};
+  for (let i = 0; i < n; i++) {
+    const c = `${id}-c${i}`;
+    const t = `${id}-t${i}`;
+    nodes[c] = node('Container', { tag: 'div', style: { padding: '8px' } }, [t], { parent: id });
+    nodes[t] = node('TextBlock', { text: `${LONG} (${id} col ${i + 1})`, style: { fontSize: '16px' } }, [], { parent: c });
+    linked[`col-${i}`] = c;
+  }
+  nodes[id] = node('ColumnLayout', { columns: n, gap: '16px', anchorId: id, ...props }, [], { parent: 'ROOT', linkedNodes: linked });
+}
+
+export const NAV_LINKS = ['Home', 'About us', 'Services', 'Our work', 'Pricing', 'Blog', 'Contact'].map((text, i, a) => ({
+  text, href: '#' + text.toLowerCase().replace(/\s/g, '-'), ...(i === a.length - 1 ? { isCta: true } : {}),
+}));
+
+export function syntheticState(navShowMobileMenu: boolean | undefined): string {
+  const nodes: Record<string, any> = {};
+  const kids: string[] = ['nav'];
+  const navProps: Record<string, unknown> = { logoText: 'Acme Plumbing & Heating', links: NAV_LINKS, padding: '16px 24px' };
+  if (navShowMobileMenu !== undefined) navProps.showMobileMenu = navShowMobileMenu;
+  nodes.nav = node('Navbar', navProps, [], { parent: 'ROOT' });
+  const specs: Array<[string, Record<string, unknown>, number]> = [
+    ['c2', { split: '50-50' }, 2],
+    ['centered', { split: '50-50', gap: '48px', style: { alignItems: 'center' } }, 2],
+    ['c3', { split: '33-33-33' }, 3],
+    ['c4', { split: '25-25-25-25' }, 4],
+    ['c6', { split: '16-16-16-16-16-16' }, 6],
+    ['sideL', { split: '30-70' }, 2],
+    ['sideR', { split: '70-30' }, 2],
+    ['optout', { split: '50-50', styleMobile: { flexDirection: 'row' } }, 2],
+  ];
+  for (const [id, p, n] of specs) { addColumns(nodes, id, p, n); kids.push(id); }
+  // An existing author-built flex row (already stacks via the global rule) must be unaffected.
+  nodes.row = node('Container', { tag: 'div', style: { display: 'flex', flexDirection: 'row', gap: '12px' } }, ['rt1', 'rt2'], { parent: 'ROOT' });
+  nodes.rt1 = node('TextBlock', { text: 'Row item one', style: {} }, [], { parent: 'row' });
+  nodes.rt2 = node('TextBlock', { text: 'Row item two', style: {} }, [], { parent: 'row' });
+  kids.push('row');
+  nodes.ROOT = node('Container', { tag: 'div', style: {} }, kids);
+  return JSON.stringify(nodes);
+}
+
+export function renderSynthetic(navShowMobileMenu: boolean | undefined): string {
+  const state = syntheticState(navShowMobileMenu);
+  const body = exportBodyHtml(state).html;
+  return exportComposedPreviewHtml(state, body, { title: 'Synthetic', includeFonts: false });
+}
+
+/** Stored OLS page rendered by the CURRENT frontend: regenerated body + current head. */
+export function renderStored(project: any, pageIdx: number): string {
+  const page = project.pages_craft_state[pageIdx];
+  const hdr = project.header_craft_state ? exportBodyHtml(project.header_craft_state, 'header').html : '';
+  const ftr = project.footer_craft_state ? exportBodyHtml(project.footer_craft_state, 'footer').html : '';
+  const body = exportBodyHtml(page.craftState).html;
+  return exportComposedPreviewHtml(page.craftState, hdr + body + ftr, { title: page.name, includeFonts: true, headCode: project.design?.headCode, design: project.design });
+}
+
+/** The checked-in (OLD) stored body under the CURRENT head: "existing site, next publish". */
+export function renderOldBodyNewHead(project: any, storedHtml: string): string {
+  const m = storedHtml.match(/<body[^>]*>([\s\S]*)<\/body>/i);
+  const body = m ? m[1] : storedHtml;
+  return exportComposedPreviewHtml(project.pages_craft_state[0].craftState, body, { title: 'old', includeFonts: true, design: project.design });
+}
diff --git a/craft/scripts/e2e-mobile-parity.mjs b/craft/scripts/e2e-mobile-parity.mjs
new file mode 100644
index 0000000..898c06a
--- /dev/null
+++ b/craft/scripts/e2e-mobile-parity.mjs
@@ -0,0 +1,131 @@
+#!/usr/bin/env node
+/**
+ * Real-browser regression test (Firefox AND Chromium, headless): published/Preview documents must
+ * stack multi-column layouts at phone width, must not overflow horizontally at 390px, and must lay
+ * out EXACTLY as before at desktop width.
+ *
+ *   npm run test:browser:mobile
+ *   MOBILE_PARITY_RECORD=<file>   write a layout snapshot (run on the base commit to get a baseline)
+ *   MOBILE_PARITY_BASELINE=<file> additionally assert desktop (1280) boxes equal that snapshot
+ *
+ * Env: CHROMIUM_PATH / FIREFOX_PATH, MOBILE_ONLY=chromium|firefox.
+ */
+import { chromium, firefox } from '@playwright/test';
+import { build as esbuild } from 'esbuild';
+import fs from 'node:fs';
+import os from 'node:os';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const root = path.resolve(path.dirname(fileURLToPath(import.meta.url)), '..');
+const fixDir = path.join(root, 'tests/fixtures/canvas-parity');
+
+const out = await esbuild({
+  entryPoints: [path.join(root, 'scripts/e2e-mobile-parity-entry.ts')],
+  bundle: true, write: false, format: 'esm', platform: 'node', jsx: 'automatic', logLevel: 'error',
+  define: { 'process.env.NODE_ENV': '"production"', __EDITOR_BUILD__: '"test"' },
+  banner: { js: "import { createRequire } from 'node:module'; const require = createRequire(import.meta.url);" },
+});
+const tmp = path.join(os.tmpdir(), `wsb-mobile-${process.pid}.mjs`);
+fs.writeFileSync(tmp, out.outputFiles[0].text);
+const ent = await import(tmp);
+fs.rmSync(tmp, { force: true });
+
+const project = JSON.parse(fs.readFileSync(path.join(fixDir, 'ols.project.json'), 'utf8'));
+const docs = {
+  'synthetic (navbar showMobileMenu unset = existing)': ent.renderSynthetic(undefined),
+  'synthetic (navbar showMobileMenu false)': ent.renderSynthetic(false),
+  'synthetic (navbar showMobileMenu true)': ent.renderSynthetic(true),
+};
+project.pages_craft_state.forEach((pg, i) => { docs[`ols/${pg.slug} (regenerated)`] = ent.renderStored(project, i); });
+for (const slug of ['index', 'about', 'contact', 'brands']) {
+  docs[`ols/${slug} (stored body, current head)`] = ent.renderOldBodyNewHead(project, fs.readFileSync(path.join(fixDir, `ols.${slug}.html`), 'utf8'));
+}
+
+function measure() {
+  const vis = (el) => { const r = el.getBoundingClientRect(); return getComputedStyle(el).display !== 'none' && (r.width > 0 || r.height > 0); };
+  const els = [...document.body.querySelectorAll('*')].filter((e) => !['SCRIPT', 'STYLE', 'NOSCRIPT'].includes(e.tagName));
+  const boxes = els.map((el, i) => {
+    const r = el.getBoundingClientRect();
+    return [i, el.tagName, Math.round(r.x * 10) / 10, Math.round(r.y * 10) / 10, Math.round(r.width * 10) / 10, Math.round(r.height * 10) / 10, getComputedStyle(el).display];
+  });
+  let maxRight = 0; let worst = '';
+  for (const el of els) { if (!vis(el)) continue; const r = el.getBoundingClientRect(); if (r.right > maxRight) { maxRight = r.right; worst = el.tagName + '.' + (el.className || '') + ':' + (el.textContent || '').slice(0, 20); } }
+  // Multi-column layouts: ColumnLayout wrappers (class cols_*) -> do their column children share a row?
+  const cols = [...document.querySelectorAll('[class^="cols_"]')].map((w) => {
+    const kids = [...w.children].filter((c) => !['STYLE', 'SCRIPT'].includes(c.tagName));
+    const tops = new Set(kids.map((k) => Math.round(k.getBoundingClientRect().top)));
+    return { id: w.id, n: kids.length, rows: tops.size, dir: getComputedStyle(w).flexDirection, minW: Math.min(...kids.map((k) => k.getBoundingClientRect().width)) };
+  });
+  const rowEls = [...document.querySelectorAll('div')].filter((d) => /flex-direction:\s*row/.test(d.getAttribute('style') || '') && /display:\s*flex/.test(d.getAttribute('style') || '') && !/^cols_/.test(d.className));
+  const rows = rowEls.map((d) => getComputedStyle(d).flexDirection);
+  const nav = document.querySelector('nav');
+  let navRight = 0;
+  if (nav) for (const el of [nav, ...nav.querySelectorAll('*')]) { if (vis(el)) navRight = Math.max(navRight, el.getBoundingClientRect().right); }
+  const navInfo = nav ? { w: nav.getBoundingClientRect().width, h: Math.round(nav.getBoundingClientRect().height), scrollW: nav.scrollWidth, hamburger: !!nav.querySelector('.navbar-hamburger') && getComputedStyle(nav.querySelector('.navbar-hamburger')).display } : null;
+  return { innerW: window.innerWidth, maxRight: Math.round(maxRight * 10) / 10, navRight: Math.round(navRight * 10) / 10, worst, cols, rows, navInfo, boxes };
+}
+
+const results = {};
+let fail = 0;
+const ok = (c, m) => { if (!c) { fail++; console.log('  FAIL', m); } };
+const baseline = process.env.MOBILE_PARITY_BASELINE ? JSON.parse(fs.readFileSync(process.env.MOBILE_PARITY_BASELINE, 'utf8')) : null;
+
+async function run(name, launcher, exe) {
+  const browser = await launcher.launch({ executablePath: exe || undefined, ...(name === 'chromium' ? { args: ['--no-sandbox'] } : {}) });
+  results[name] = {};
+  for (const [docName, html] of Object.entries(docs)) {
+    results[name][docName] = {};
+    for (const w of [390, 1280]) {
+      const page = await browser.newPage({ viewport: { width: w, height: 900 } });
+      // Deterministic: no network (web fonts / CDN icons would race the measurement).
+      await page.route(/^https?:/, (r) => r.abort());
+      await page.setContent(html, { waitUntil: 'load' });
+      await page.waitForTimeout(50);
+      const m = await page.evaluate(measure);
+      await page.close();
+      results[name][docName][w] = m;
+      const tag = `[${name} ${w}] ${docName}`;
+      if (w === 390) {
+        // ols/index carries a customer HtmlBlock card grid that overflows on its own (out of scope here): for it,
+        // only the navbar is held to the phone width.
+        ok(m.navRight <= 390.5, `${tag}: navbar overflows, reaches x=${m.navRight}`);
+        if (!docName.startsWith('ols/index')) ok(m.maxRight <= 390.5, `${tag}: horizontal overflow, content reaches x=${m.maxRight} (${m.worst})`);
+        for (const c of m.cols) {
+          const optOut = c.id === 'optout';
+          if (optOut) ok(c.rows === 1 || c.n === 1, `${tag}: opt-out columns should stay side by side (${JSON.stringify(c)})`);
+          else ok(c.rows === c.n, `${tag}: columns not stacked (${JSON.stringify(c)})`);
+        }
+      }
+      if (w === 1280 && baseline) {
+        const b = baseline[name]?.[docName]?.[1280];
+        if (b) {
+          const sameCount = b.boxes.length === m.boxes.length;
+          ok(sameCount, `${tag}: element count ${b.boxes.length} -> ${m.boxes.length}`);
+          let diffs = 0;
+          if (sameCount) for (let i = 0; i < b.boxes.length; i++) {
+            const bb = b.boxes[i]; const mm = m.boxes[i];
+            if (bb.slice(2, 6).some((v, j) => Math.abs(v - mm[2 + j]) > 0.5) || bb[6] !== mm[6]) { if (diffs++ < 3) console.log('   diff', JSON.stringify(bb), JSON.stringify(mm)); }
+          }
+          ok(diffs === 0, `${tag}: ${diffs} element boxes changed at desktop`);
+        }
+      }
+    }
+  }
+  await browser.close();
+}
+
+const only = process.env.MOBILE_ONLY;
+if (!only || only === 'chromium') await run('chromium', chromium, process.env.CHROMIUM_PATH);
+if (!only || only === 'firefox') await run('firefox', firefox, process.env.FIREFOX_PATH);
+
+if (process.env.MOBILE_PARITY_RECORD) fs.writeFileSync(process.env.MOBILE_PARITY_RECORD, JSON.stringify(results));
+// Human-readable summary at 390.
+for (const [b, per] of Object.entries(results)) {
+  for (const [d, r] of Object.entries(per)) {
+    const m = r[390];
+    console.log(`${b.padEnd(8)} 390 ${d.padEnd(55)} maxRight=${String(m.maxRight).padEnd(7)} cols(rows/n)=${m.cols.map((c) => c.rows + '/' + c.n).join(',') || '-'} nav=${m.navInfo ? m.navInfo.h + 'h/' + m.navInfo.hamburger + '/right=' + m.navRight : '-'}`);
+  }
+}
+console.log(fail ? `\n${fail} FAILED` : '\nall passed');
+process.exit(fail ? 1 : 0);