LayoutTests/webanimations/opacity-animation.html

<div style="position: absolute; top: 0; left: 0; width: 100px; height: 100px; background-color: black;"></div>
<script>

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

const animation = document.querySelector("div").animate([
    { opacity: 0 },
    { opacity: 0.5, offset: 0.001 },
    { opacity: 0.5 }
], 1000);

if (window.testRunner) {
    animation.ready.then(() => {
        setTimeout(() => testRunner.notifyDone(), 100);
    });
}

</script>