chromium/chrome/test/data/page_load_metrics/preact_page.html

<!DOCTYPE html>

<body>
  <div id="nothing"></div>
  <div id="container">
    <div id="root">
      This is a test to see if a page with a div with __k property is
      detected as using the Preact JavaScript framework.
    </div>
  </div>
  <script>
    document.getElementById('root').__k = "test";
  </script>
</body>