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chromium/third_party/blink/web_tests/fast/events/resources/mouseover-button-iframe.html 

<body style="margin-left:0; margin-top:0">
    <script>
        function doOnMouseEvent(evt)
        {
            top.document.events[top.document.saveWhere] = evt;
        }

    </script>
    <div style="height: 50; width: 50; background:violet; top:0;left:0; position:absolute;"
        onMouseOver="doOnMouseEvent(event);"
        >1</div>
</body>