LayoutTests/webanimations/accelerated-overlapping-transform-animations.html

<style>

    #target {
        position: absolute;
        top: 0;
        left: 0;
        width: 100px;
        height: 100px;
        background-color: black;
    }

</style>
<div id="target"></div>
<script src="../resources/ui-helper.js"></script>
<script>

const waitForAnimationCommit = async (animation) => {
    await animation.ready;
    if (!UIHelper.isWebKit2()) {
        await UIHelper.renderingUpdate();
        await UIHelper.renderingUpdate();
    } else
        await UIHelper.ensureStablePresentationUpdate()
}

(async () => {
    if (window.testRunner)
        testRunner.waitUntilDone();

    const target = document.getElementById("target");

    // Make the animations last a whole minute so they don't end while the test is running. 
    const duration = 60 * 1000;

    // Start a first stationary "transform" animation and wait until it has been applied before starting
    // a second stationary "transform" animation which should replace the first one.
    await waitForAnimationCommit(target.animate({ transform: ["translateX(100px)", "translateX(100px)"] }, duration));
    await waitForAnimationCommit(target.animate({ transform: ["translateX(200px)", "translateX(200px)"] }, duration));

    if (window.testRunner)
        testRunner.notifyDone();
})();

</script>