<!DOCTYPE html>
<html>
<head>
<style>
  html, body {
    margin: 0;
    padding: 0;
  }

  video {
    width: 320px;
    height: 240px;
  }

  #mask {
    position: absolute;
    z-index: 3;
    width: 320px;
    height: 200px;
    background-color: green;
    top: 0;
    left: 0;
  }
</style>
</head>
<body>
  <video id="vid" controls>
    <track kind="subtitles" src="foo.vtt" srclang="en">
  </video>
  <div id="mask"></div>
</body>
</html>
