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chromium/third_party/blink/web_tests/fast/frames/resources/sandboxed-iframe-attribute-parsing-allowed.html 

<html>
<script>

top.allowedCallFromSandbox();

</script>
<body onload="document.getElementById('f').submit();">
    <form id='f' action="javascript:top.disallowedFormSubmitted();">
        <input name="x" value="y" type="checkbox" checked="yes" />
    </form>
</body>
</html>