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chromium
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chromium/third_party/blink/web_tests/fast/events/resources/page-visibility-iframe-with-subframes.html 

<html>
<body onload="parent.startTest()">
<iframe id="subIframe1" onload="parent.parent.startTest()"></iframe>
<iframe id="subIframe2" onload="parent.parent.startTest()"></iframe>
</body>
</html>