code: function setElement( targetElement ) {
      // If the element has already been set,
      // clear the transition classes from the old element.
      if ( _dom ) {
        remove();
        animateOn();
      }
      _dom = targetElement;
      _dom.classList.add( _classes.BASE_CLASS );
      _transitionEndEvent = _getTransitionEndEvent( _dom );
    }
docs: Set the HTML element target of this transition.
  @param {HTMLNode} targetElement - The target of the transition.