Codebase Browser
chromium
Go to App

chromium/third_party/blink/web_tests/media/controls/add-poster-after-page-load.html 

<!DOCTYPE html>
<html>
<title>Test that adding a poster after page load shows the poster.</title>
<script src="../../resources/testharness.js"></script>
<script src="../../resources/testharnessreport.js"></script>
<script src="../media-controls.js"></script>
<video controls width=400 preload=none src="../content/60_sec_video.webm"></video>
<script>
async_test(t => {
  const video = document.querySelector('video');

  window.onload = t.step_func(() => {
    assert_true(showsDefaultPoster());
    video.setAttribute('poster', '../content/greenbox.png');
    setTimeout(t.step_func_done(() => {
      assert_false(showsDefaultPoster());
    }));
  });

  function showsDefaultPoster() {
    return mediaControls(video).classList.contains('use-default-poster');
  }
});
</script>
</html>