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chromium/chrome/test/data/media/engagement/engagement_test_iframe_child.html 

<!DOCTYPE html>
<meta charset="utf-8">
<title>Media Engagement Test</title>
<video autoplay src="engagement_valid_video.webm" id="video"></video>
<script>
document.getElementById("video").onplay = function() {
  parent.postMessage("Ready", "*");
};
</script>