chromium/chrome/test/data/extensions/platform_apps/web_view/shim/fenced_frame_host.html

<!DOCTYPE html>
<html>
<body>
  <p>This document hosts a fenced frame:</p>
  <fencedframe></fencedframe>
  <script>
    const url = new URL("fenced_frame.html", location.href);
    document.querySelector("fencedframe").config = new FencedFrameConfig(url);
  </script>
</body>
</html>