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chromium/third_party/blink/web_tests/external/wpt/client-hints/sandbox/resources/iframe-with-embedded-popup-expect-no-hints.html 

<!DOCTYPE html>
<html>
<body>
<script>
  window.addEventListener('message', e => {
    window.parent.postMessage(e.data, '*');
  });

</script>
<iframe src="embedded-popup-expect-no-hints.html"></iframe>
</body>
</html>