<!DOCTYPE html>

<html>
<head>
    <style>
        .box {
          width: 200px;
          height: 200px;
          margin: 10px;
          background-color: blue;
        }
        
        .composited {
          -webkit-transform: translateZ(0);
        }
        
        .clipped {
            -webkit-clip-path: inset(0 30px);
        }
        
        .masked {
            -webkit-mask: linear-gradient(transparent, black);
        }
        
    </style>
    <script>
        if (window.testRunner)
            testRunner.waitUntilDone();

        function doTest()
        {
            window.setTimeout(function() {
                document.getElementById('gain-mask').classList.add('masked');
                document.getElementById('lose-mask').classList.remove('masked');
                if (window.testRunner)
                    testRunner.notifyDone();
            }, 0);
        }
        
        window.addEventListener('load', doTest, false);
    </script>
</head>
<body>

<div id="gain-mask" class="clipped composited box"></div>
<div id="lose-mask" class="masked clipped composited box"></div>

</body>
</html>