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

#target {
    position: absolute;
    top: 0;
    left: 0;
    width: 100px;
    height: 100px;
    background-color: black;
    animation: anim1 50ms forwards, anim2 5s 50ms;
}

@keyframes anim1 {
    0%, 100% { background-color: red; }
    50% { background-color: orange; }
}

@keyframes anim2 {
    0% { transform: translateX(0) }
    5%, 100% { transform: translateX(500px) }
}

body {
    background-color: white;
}

#cache > div {
    position: absolute;
    background-color: white;
}

#cache > div:nth-of-type(1) {
    top: 0;
    left: 100px;
    right: 0;
    height: 100%;
}

#cache > div:nth-of-type(2) {
    top: 100px;
    left: 0;
    bottom: 0;
    width: 100%;
}

</style>
</head>
<body>
<div id="target"></div>
<div id="cache">
    <div></div>
    <div></div>
</div>
<script>

if (window.testRunner) {
    testRunner.waitUntilDone();
    document.getElementById("target").addEventListener("animationstart", event => {
        if (event.animationName == "anim2")
            setTimeout(() => testRunner.notifyDone(), 100);
    });
}

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