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chromium/third_party/blink/web_tests/external/wpt/fullscreen/api/resources/report-fullscreen-enabled.html 

<!DOCTYPE html>
<meta charset="utf-8">
<title>IFrame fullscreenEnabled attribute reporter</title>
<body>
<script>
parent.postMessage({"report": {
  "api": "fullscreen",
  "enabled": document.fullscreenEnabled,
  "frame": window.name
}}, "*");
</script>