platform/video.videojs/dist/video.es.js
function addClass(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 (!hasClass(element, classToAdd)) {
    element.className = (element.className + ' ' + classToAdd).trim();
  }

  return element;
}
function addClass(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 (!hasClass(element, classToAdd)) {
    element.className = (element.className + ' ' + classToAdd).trim();
  }

  return element;
}
Add a CSS class name to an element

@param {Element} element
Element to add class name to.

@param {string} classToAdd
Class name to add.

@return {Element}
The dom element with the added class name.