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

div {
    left: 0;
    top: 0;
    position: absolute;
    height: 100px;
}

.mask {
    /* We add an extra pixel to account for rounding errors */
    width: 51px;
    background-color: red;
}

#target {
    width: 100px;
    background-color: black;
}

#right {
    left: 149px;
}

</style>
</head>
<body>

<div id="target"></div>
<div id="left" class="mask"></div>
<div id="right" class="mask"></div>

<script>

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

const animation = document.getElementById("target").animate({
    transform: ["translateX(100px)", "none"]
}, { duration: 10000, delay: 1000 });

requestAnimationFrame(() => {
    animation.pause();
    animation.currentTime = 6000;
    if (window.testRunner) {
        requestAnimationFrame(() => {
            requestAnimationFrame(() => testRunner.notifyDone())
        });
    }
});

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