LayoutTests/webanimations/set-keyframes-after-animation-completion.html

<style>

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

</style>
<div id="target"></div>
<script>

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

    const timing = { fill: "forwards", duration: 1 };
    const originalKeyframes = { marginLeft: ["0px", "100px"] };
    const updatedKeyframes = { marginLeft: ["50px", "150px"] };
    const animation = document.getElementById("target").animate(originalKeyframes, timing);

    await animation.finished;
    await new Promise(resolve => setTimeout(resolve));

    animation.effect.setKeyframes(updatedKeyframes);

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

</script>