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chromium/third_party/blink/web_tests/fullscreen/resources/full-screen-iframe-allowed-nested1.html 

<!DOCTYPE html>
iframe1
<iframe allowfullscreen src="full-screen-iframe-allowed-nested2.html">
</iframe>
<script>
window.onmessage = function (messageEvent) {
    parent.postMessage(messageEvent.data, "*");
}
</script>