docs: function addClass (el, cls) {
  	  /* istanbul ignore else */
  	  if (el.classList) {
  	    if (cls.indexOf(' ') > -1) {
  	      cls.split(/\s+/).forEach(function (c) { return el.classList.add(c); });
  	    } else {
  	      el.classList.add(cls);
  	    }
  	  } else {
  	    var cur = ' ' + el.getAttribute('class') + ' ';
  	    if (cur.indexOf(' ' + cls + ' ') < 0) {
  	      el.setAttribute('class', (cur + cls).trim());
  	    }
  	  }
  	}