docs: function () {
          var cleanup = function () {
              u.off(this.element,'transitionend', cleanup);
              // this.element.removeEventListener('transitionend', cleanup);
              u.off(this.element,'webkitTransitionEnd', cleanup);
              // this.element.removeEventListener('webkitTransitionEnd', cleanup);
              u.removeClass(this.element, 'is-animating');
          }.bind(this);
  
          // Remove animation class once the transition is done.
          u.on(this.element,'transitionend', cleanup);
          // this.element.addEventListener('transitionend', cleanup);
          u.on(this.element,'webkitTransitionEnd', cleanup);
          // this.element.addEventListener('webkitTransitionEnd', cleanup);
      }