chromium/chrome/test/data/web_app_file_handling/basic_app.html

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <link rel="manifest" href="basic_app_manifest.json">
</head>
<body>
  <h1>Web app that handles file opening</h1>
  <script>
    window.onload = () => {
      navigator.serviceWorker.register('/web_app_file_handling/service_worker.js');
    };
  </script>
</body>
</html>