tabindex="-1"])') &&
          this._isVisible(element);
      },

      /**
       * Returns the normalized element tabindex. If not focusable, returns -1.
       * It checks for the attribute "tabindex" instead of the element property
       * `tabIndex` since browsers assign different values to it.
       * e.g. in Firefox `<div contenteditable>` has `tabIndex = -1`
       * @param {!HTMLElement} element
       * @return {!number}
       * @private
       */
      _normalizedTabIndex: function(element) {