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chromium/chrome/test/data/reduce_accept_language/cross_origin_fenced_frame.html 

<html>
<link rel="icon" href="data:;base64,=">
<body>
<fencedframe></fencedframe>
<script>
  const url = "https://my-site.com:44444/subframe_simple_3p.html";
  document.querySelector("fencedframe").config = new FencedFrameConfig(url);
</script>
</body>
</html>