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chromium/chrome/test/data/media/picture-in-picture/player_metadata_poster.html 

<!DOCTYPE html>
<video preload=metadata poster='../../banners/image-512px.png' src='../bigbuck.webm'></video>
<script>
  async function enterPictureInPicture() {
    await document.querySelector('video').requestPictureInPicture();
    return true;
  }
</script>