static getTransitionEvent(optEl) {
    if (transitionEvent && !optEl) {
      return transitionEvent;
    }

    transitionEvent = '';

    const el = optEl || document.createElement('ws');
    const transitions = {
      'transition': 'transitionend',
      'OTransition': 'oTransitionEnd',
      'MozTransition': 'transitionend',
      'WebkitTransition': 'webkitTransitionEnd'
    };
    const transitionNames = Object.keys(transitions);

    for (let i = 0, length = transitionNames.length;
          i < length && !transitionEvent; i++) {
      const transitionName = transitionNames[i];

      if (typeof el.style[transitionName] !== 'undefined') {
        transitionEvent = transitions[transitionName];
      }
    }

    return transitionEvent;
  }
Gets the prefixed transitionend event.
@param {?Element} optEl Element to check
@return {string}