diff --git a/tests/playwright/isomorphic.spec.js b/tests/playwright/isomorphic.spec.js
new file mode 100644
index 0000000..11042f5
--- /dev/null
+++ b/tests/playwright/isomorphic.spec.js
@@ -0,0 +1,177 @@
+// @ts-check
+const { test, expect } = require('playwright/test');
+
+const BASE_URL = process.env.SX_TEST_URL || 'http://localhost:8013';
+const TEST_PAGE = '/sx/(etc.(philosophy.wittgenstein))';
+
+/**
+ * Helper: get the text content of #sx-root, normalised.
+ * Strips whitespace differences so SSR and client render can be compared.
+ */
+async function getSxRootText(page) {
+  return page.evaluate(() => {
+    const root = document.getElementById('sx-root');
+    if (!root) return '';
+    return root.innerText.replace(/\s+/g, ' ').trim();
+  });
+}
+
+/**
+ * Helper: get structural snapshot of #sx-root — tag names, ids, classes, text.
+ * Ignores attributes that differ between SSR and client (event handlers, etc).
+ */
+async function getSxRootStructure(page) {
+  return page.evaluate(() => {
+    function snapshot(el) {
+      if (el.nodeType === 3) {
+        const text = el.textContent.trim();
+        return text ? { t: text } : null;
+      }
+      if (el.nodeType !== 1) return null;
+      const node = { tag: el.tagName.toLowerCase() };
+      if (el.id) node.id = el.id;
+      // Collect class names, sorted for stability
+      const cls = Array.from(el.classList).sort().join(' ');
+      if (cls) node.cls = cls;
+      // Data attributes for islands and lakes
+      const island = el.getAttribute('data-sx-island');
+      if (island) node.island = island;
+      const lake = el.getAttribute('data-sx-lake');
+      if (lake) node.lake = lake;
+      // Recurse children
+      const children = [];
+      for (const child of el.childNodes) {
+        const s = snapshot(child);
+        if (s) children.push(s);
+      }
+      if (children.length) node.children = children;
+      return node;
+    }
+    const root = document.getElementById('sx-root');
+    return root ? snapshot(root) : null;
+  });
+}
+
+// ---------------------------------------------------------------------------
+// Tests
+// ---------------------------------------------------------------------------
+
+test.describe('Isomorphic SSR', () => {
+
+  test('page renders visible content without JavaScript', async ({ browser }) => {
+    const context = await browser.newContext({ javaScriptEnabled: false });
+    const page = await context.newPage();
+    await page.goto(BASE_URL + TEST_PAGE, { waitUntil: 'domcontentloaded' });
+
+    // sx-root should exist and have content
+    const root = page.locator('#sx-root');
+    await expect(root).toBeVisible();
+
+    // Should have real HTML content (headings from the article)
+    const headings = await page.locator('#sx-root h2').allTextContents();
+    expect(headings.length).toBeGreaterThan(0);
+    expect(headings[0]).toContain('Language games');
+
+    // Header island should be rendered with hydration marker
+    const headerIsland = page.locator('[data-sx-island="layouts/header"]');
+    await expect(headerIsland).toBeVisible();
+
+    // Logo should be visible
+    await expect(page.locator('#sx-root').getByText('(<sx>)')).toBeVisible();
+
+    // Copyright should show the path
+    await expect(page.locator('#sx-root').getByText('© Giles Bradshaw 2026')).toBeVisible();
+
+    await context.close();
+  });
+
+  test('page renders with JavaScript and matches SSR structure', async ({ browser }) => {
+    // First: get SSR content (no JS)
+    const noJsContext = await browser.newContext({ javaScriptEnabled: false });
+    const noJsPage = await noJsContext.newPage();
+    await noJsPage.goto(BASE_URL + TEST_PAGE, { waitUntil: 'domcontentloaded' });
+    const ssrText = await getSxRootText(noJsPage);
+    const ssrStructure = await getSxRootStructure(noJsPage);
+    await noJsContext.close();
+
+    // Then: get client-rendered content (with JS)
+    const jsContext = await browser.newContext({ javaScriptEnabled: true });
+    const jsPage = await jsContext.newPage();
+    await jsPage.goto(BASE_URL + TEST_PAGE, { waitUntil: 'networkidle' });
+    // Wait for islands to hydrate
+    await jsPage.waitForSelector('[data-sx-island].sx-processed', { timeout: 10000 }).catch(() => {});
+    await jsPage.waitForTimeout(500); // settle
+
+    const clientText = await getSxRootText(jsPage);
+    await jsContext.close();
+
+    // Text content should be the same (article text, headings, etc)
+    expect(ssrText.length).toBeGreaterThan(100);
+    expect(clientText.length).toBeGreaterThan(100);
+
+    // The core content should match — both should have the article headings
+    expect(clientText).toContain('Language games');
+    expect(clientText).toContain('The limits of my language');
+
+    // SSR should also have these
+    expect(ssrText).toContain('Language games');
+    expect(ssrText).toContain('The limits of my language');
+  });
+
+  test('header island has CSSX styling without JavaScript', async ({ browser }) => {
+    const context = await browser.newContext({ javaScriptEnabled: false });
+    const page = await context.newPage();
+    await page.goto(BASE_URL + TEST_PAGE, { waitUntil: 'domcontentloaded' });
+
+    // The logo should have the violet color class
+    const logo = page.locator('[data-sx-island="layouts/header"] span.sx-text-violet-699');
+    await expect(logo).toBeVisible();
+
+    // Check that the CSSX style tag is in <head>
+    const cssxInHead = await page.evaluate(() => {
+      const style = document.querySelector('head style[data-cssx]');
+      return style ? style.textContent.length : 0;
+    });
+    expect(cssxInHead).toBeGreaterThan(0);
+
+    // The violet rule should exist
+    const hasVioletRule = await page.evaluate(() => {
+      const style = document.querySelector('head style[data-cssx]');
+      return style ? style.textContent.includes('sx-text-violet-699') : false;
+    });
+    expect(hasVioletRule).toBe(true);
+
+    await context.close();
+  });
+
+  test('navigation preserves header island state', async ({ page }) => {
+    await page.goto(BASE_URL + '/sx/', { waitUntil: 'networkidle' });
+
+    // Wait for header island to hydrate
+    await page.waitForSelector('[data-sx-island="layouts/header"]', { timeout: 15000 });
+    await page.waitForTimeout(1000);
+
+    // Click "reactive" to change colour
+    const reactive = page.locator('[data-sx-island="layouts/header"]').getByText('reactive');
+    await reactive.click();
+    await page.waitForTimeout(300);
+
+    // Get the colour after click
+    const colourBefore = await reactive.evaluate(el => el.style.color);
+    expect(colourBefore).toBeTruthy();
+
+    // Navigate via SPA link
+    const geoLink = page.locator('a[sx-get*="geography"]').first();
+    await geoLink.click();
+    await page.waitForTimeout(2000);
+
+    // Colour should be preserved (island not disposed)
+    const colourAfter = await reactive.evaluate(el => el.style.color);
+    expect(colourAfter).toBe(colourBefore);
+
+    // Copyright path should update
+    const copyright = page.locator('[data-sx-lake="copyright"]');
+    await expect(copyright).toContainText('geography');
+  });
+
+});
diff --git a/tests/playwright/playwright.config.js b/tests/playwright/playwright.config.js
new file mode 100644
index 0000000..0911b29
--- /dev/null
+++ b/tests/playwright/playwright.config.js
@@ -0,0 +1,15 @@
+// @ts-check
+const { defineConfig } = require('playwright/test');
+
+module.exports = defineConfig({
+  testDir: '.',
+  timeout: 60000,
+  retries: 0,
+  use: {
+    baseURL: process.env.SX_TEST_URL || 'http://localhost:8013',
+    headless: true,
+  },
+  projects: [
+    { name: 'chromium', use: { browserName: 'chromium' } },
+  ],
+});