chromium/third_party/blink/web_tests/media/video-poster-after-loadedmetadata.html

<!DOCTYPE html>
<title>Tests setting the poster attribute after a video fires loadedmetadata.</title>
<video></video>
<script>
if (window.testRunner)
  testRunner.waitUntilDone();

var video = document.querySelector('video');
video.addEventListener("loadedmetadata", _ => {
  video.poster = "content/abe.png";
  setTimeout(_ => requestAnimationFrame(_ => testRunner.notifyDone()));
});

video.src = "resources/test-positive-start-time.webm";
</script>