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chromium/third_party/blink/web_tests/external/wpt/html/semantics/embedded-content/the-iframe-element/resources/unload-reporter.html 

<!DOCTYPE html>
<h1>I'll report to my parent when I'm unloaded</h1>

<script>
  window.onbeforeunload = e => {
    parent.postMessage('unloading', '*');
  };
</script>