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chromium/chrome/test/data/extensions/api_test/executescript/fenced_frames/main.html 

<html>
<fencedframe></fencedframe>
<fencedframe></fencedframe>
<script>
  // Only 1 fenced frame is navigated. The second one stays in its default
  // non-navigated state to test that the executeScript() promise resolves if a
  // fenced frame hasn't been navigated yet.
  const url = new URL("fenced_frame.html", location.href);
  document.querySelector("fencedframe").config = new FencedFrameConfig(url);
</script>
</html>