chromium/content/test/data/media/picture_in_picture/two-videos.html

<!DOCTYPE html>
<html>
<body>
<video src="../tulip2.webm"></video>
<video src="../tulip2.webm"></video>
<iframe src="data:text/html,<video src='../tulip2.webm'></video>"></iframe>
</body>
<script>
  const videos = document.querySelectorAll('video');
  videos[0].addEventListener('playing', e => {
    document.title = 'videos[0] playing';
  });
  videos[0].addEventListener('enterpictureinpicture', e => {
    document.title = 'videos[0] entered picture-in-picture';
  });
  videos[1].addEventListener('playing', e => {
    document.title = 'videos[1] playing';
  });
  videos[1].addEventListener('enterpictureinpicture', e => {
    document.title = 'videos[1] entered picture-in-picture';
  });

  let iframeVideos = [];
  if (window.top == window.self) {
    const iframe = document.createElement('iframe');
    iframe.src = 'two-videos.html';
    document.body.appendChild(iframe);
    iframe.addEventListener('load', () => {
      document.title = 'iframe loaded';

      iframeVideos = iframe.contentDocument.querySelectorAll('video');
      iframeVideos[0].addEventListener('playing', e => {
        document.title = 'iframeVideos[0] playing';
      });
      iframeVideos[0].addEventListener('enterpictureinpicture', e => {
        document.title = 'iframeVideos[0] entered picture-in-picture';
      });
    });
  }
</script>
</html>