<!DOCTYPE html>
<html>
<head>
    <style>

        .container {
            margin-left: 100px;
            margin-top: 100px;
        }

        .animates {
            animation: fade 1ms forwards;
        }

        .child {
            position: relative;
            width: 100px;
            height: 100px;
            background-color: black;
        }

        @keyframes fade {
            from { opacity: 0; }
            to   { opacity: 1; }
        }

    </style>
</head>
<body>
    <div class="container animates">
        <div class="child"></div>
    </div>
    <script>

if (window.testRunner)
    testRunner.waitUntilDone();

document.querySelector(".container").addEventListener("animationend", event => {
    event.target.classList.remove("animates");
    if (window.testRunner)
        testRunner.notifyDone();
});

    </script>
</body>
</html>