simultaneous-reloads-assert-frame.html

<html>
<head>
    <title></title>
</head>
<body onload="parent.frameLoaded()">
    <p>
        If you do not hit an assertion failure when opening
        this test with a debug build, then you pass the test.
    </p>
    <iframe  src='data:text/html,
        <img src="somethingtolookfor.gif">
    '></iframe>
    <iframe  src='data:text/html,
        <img src="somethingtolookfor.gif">
    '></iframe>
</body>
</html>

chromium/third_party/blink/web_tests/fast/loader/resources/simultaneous-reloads-assert-frame.html