chromium/chrome/test/data/extensions/api_test/webrequest/fencedFrames/iframe.html

<script>
  onload = function() {
    var f = document.createElement('fencedframe');
    const url = new URL(
      '/server-redirect?extensions/api_test/webrequest/fencedFrames/frame.html',
      location.origin
    );
    f.config = new FencedFrameConfig(url);
    document.body.appendChild(f);
  }
</script>