<!DOCTYPE html>
<html>
<head>
    <style>
        .container {
            position: absolute;
            width: 300px;
            height: 300px;
            border: 1px solid black;
        }
        
        .box {
            position: absolute;
            width: 200px;
            height: 200px;
        }
        
        .bottom {
            left: 0px;
            top: 0px;
            background-color: orange;
        }

        .top {
            left: 100px;
            top: 100px;
            background-color: green;
            animation: fade-out 30ms forwards;
        }
        
        @keyframes fade-out {
            0% {
                opacity: 1;
            }
            100% {
                z-index: -1;
                opacity: 0.8;
            }
        }
    </style>
    <script>
        if (window.testRunner)
            testRunner.waitUntilDone();

        window.addEventListener('load', () => {
            document.getElementById('tester').addEventListener('animationend', () => {
                if (window.testRunner)
                    testRunner.notifyDone();
            }, false);
        }, false);
    </script>
</head>
<body>
    <div class="container">
        <div class="bottom box"></div>
        <div id="tester" class="top box"></div>
    </div>
</body>
</html>