diff --git a/bun.lockb b/bun.lockb
index 30fb9bf1..61d3bb85 100755
Binary files a/bun.lockb and b/bun.lockb differ
diff --git a/package.json b/package.json
index c36bd24a..fb87a1aa 100644
--- a/package.json
+++ b/package.json
@@ -28,7 +28,7 @@
     "@eslint/js": "9.17.0",
     "@maxmilton/eslint-config": "0.0.8",
     "@maxmilton/stylelint-config": "0.1.2",
-    "@maxmilton/test-utils": "0.0.6",
+    "@maxmilton/test-utils": "0.0.7",
     "@playwright/test": "1.49.1",
     "@types/bun": "1.1.14",
     "@types/chrome": "0.0.287",
diff --git a/test/e2e/fixtures.ts b/test/e2e/fixtures.ts
index 529961f2..404e87f8 100644
--- a/test/e2e/fixtures.ts
+++ b/test/e2e/fixtures.ts
@@ -28,11 +28,11 @@ export const test = baseTest.extend<{
     await context.close();
   },
   async extensionId({ context }, use) {
-    let [background] = context.serviceWorkers();
+    let [sw] = context.serviceWorkers();
     // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
-    background ??= await context.waitForEvent('serviceworker');
+    sw ??= await context.waitForEvent('serviceworker', { timeout: 200 });
 
-    const extensionId = background.url().split('/')[2];
+    const extensionId = sw.url().split('/')[2];
     await use(extensionId);
   },
 });
diff --git a/test/e2e/sw.spec.ts b/test/e2e/sw.spec.ts
index 47b20940..f25018b8 100644
--- a/test/e2e/sw.spec.ts
+++ b/test/e2e/sw.spec.ts
@@ -1,11 +1,9 @@
 import { expect, test } from './fixtures';
 
-test('background service worker', async ({ context }) => {
-  let [background] = context.serviceWorkers();
-  // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
-  background ??= await context.waitForEvent('serviceworker');
-
-  // FIXME: Better assertions
-
-  expect(background).toBeTruthy();
+test('has a single background service worker (sw.js)', async ({ context, extensionId }) => {
+  const workers = context.serviceWorkers();
+  expect(workers).toHaveLength(1);
+  expect(workers[0]?.url()).toBe(`chrome-extension://${extensionId}/sw.js`);
 });
+
+// TODO: Check there are no console messages or unhandled errors in the worker.
diff --git a/test/unit/index.test.ts b/test/unit/index.test.ts
index 343d5c2b..6621dbe6 100644
--- a/test/unit/index.test.ts
+++ b/test/unit/index.test.ts
@@ -1,5 +1,6 @@
 import { describe, expect, test } from 'bun:test';
 import { readdir } from 'node:fs/promises';
+import { validate } from '@maxmilton/test-utils/html';
 
 describe('dist files', () => {
   // FIXME: The bun file type is just inferred from the file extension, not the
@@ -34,7 +35,7 @@ describe('dist files', () => {
         expect(file.type).toBe(type); // TODO: Keep this? Type seems to be resolved from the file extension, not the file data.
       });
 
-      if (minBytes != null && maxBytes != null) {
+      if (minBytes !== undefined && maxBytes !== undefined) {
         test('is within expected file size limits', () => {
           expect.assertions(2);
           expect(file.size).toBeGreaterThan(minBytes);
@@ -49,8 +50,17 @@ describe('dist files', () => {
     const distDir = await readdir('dist');
     expect(distDir).toHaveLength(distFiles.length);
   });
+
+  test.each(distFiles.filter(([filename]) => filename.endsWith('.html')))(
+    '%s contains valid HTML',
+    async (filename) => {
+      const file = Bun.file(`dist/${filename}`);
+      const html = await file.text();
+      const result = validate(html);
+      expect(result.valid).toBeTrue();
+    },
+  );
 });
 
-// TODO: HTML files should be valid HTML
 // TODO: HTML files have correct title
 // TODO: HTML files have correct JS and CSS file references