docs: function(selector, opt_offset, opt_delay) {
    this.selector = selector;
    this.offset = opt_offset || null;
    this.delay = opt_delay || 0;
  
    // Ensure all videos are loaded.
    var els = document.querySelectorAll(this.selector);
    [].forEach.call(els, function(el) {
      el.load();
    });
  }