<!DOCTYPE html>

<html>
<head>
    <style>
        .box {
            height: 100px;
            width: 100px;
            margin: 30px;
            background-color: blue;
        }
        
        .shadowed {
            box-shadow: 10px 10px 20px black;
        }
    </style>

    <script>
        if (window.testRunner)
            testRunner.dumpAsText();

        function doTest() 
        {
            if (!window.testRunner || !window.internals)
                return;

            window.internals.startTrackingRepaints();
            document.getElementById('box').classList.add('shadowed');
            document.getElementById('repaints').textContent = window.internals.repaintRectsAsText();
            window.internals.stopTrackingRepaints();
        }
        
        window.addEventListener('load', doTest, false);
    </script>
</head>
<body>
    
    <div id="box" class="box">
    </div>

<h2>Repaint rects:</h2>
<pre id="repaints"></pre>
</body>
</html>