gecko/toolkit/content/tests/widgets/videomask.css

html, body {
  margin: 0;
  padding: 0;
}

audio, video {
  width: 140px;
  height: 100px;
  background-color: black;
}

/**
 * Create a mask for the video direction tests which covers up the throbber.
 */
#mask {
  position: absolute;
  z-index: 3;
  width: 140px;
  height: 72px;
  background-color: green;
  top: 0;
  right: 0;
}