docs: function addElClass(element, classToAdd) {
    if (element.classList) {
      element.classList.add(classToAdd);
  
      // Don't need to `throwIfWhitespace` here because `hasElClass` will do it
      // in the case of classList not being supported.
    } else if (!hasElClass(element, classToAdd)) {
      element.className = (element.className + ' ' + classToAdd).trim();
    }
  
    return element;
  }