{
        width: 100%;
        opacity: 0.0;
      }
    }
  </style>

  <script>
  /**
   * Perform basic validation of the structure of a notification from the content
   * frame.
   *
   * Returns the input if valid or `null` otherwise.
   */
  function validateContentFrameMessage(data) {
    if (!data || typeof data.type !== 'string') {
      return null;
    }
    if (data.type === 'metadatachange' && data.metadata) {
      return data;
    } else {
      return null;
    }
  }

  // Listen for events 