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chromium/third_party/blink/web_tests/external/wpt/html/browsers/browsing-the-web/overlapping-navigations-and-traversals/resources/nav-cancelation-2-helper.html 

<!DOCTYPE html>
<meta charset="utf-8">
<title>Page with child frame that navigates slowly</title>

<!--
  This file is used by `../nav-cancelation-2.sub.html`. The iframe below is its
  grandchild iframe, and whenever its load event fires we report this up to our
  parent Document.
-->
<iframe src="slow.py"></iframe>

<script>
  window.parent.postMessage("grandchild frame created", "*");
  const iframe = document.querySelector('iframe');
  iframe.onload = e => {
    window.parent.postMessage("grandchild frame loaded", "*");
  };
</script>