code: function upgradeElementsInternal(elements) {
      if (!Array.isArray(elements)) {
        if (elements instanceof Element) {
          elements = [elements];
        } else {
          elements = Array.prototype.slice.call(elements);
        }
      }
      for (var i = 0, n = elements.length, element; i < n; i++) {
        element = elements[i];
        if (element instanceof HTMLElement) {
          upgradeElementInternal(element);
          if (element.children.length > 0) {
            upgradeElementsInternal(element.children);
          }
        }
      }
    }
docs: Upgrades a specific list of elements rather than all in the DOM.
  
  @param {!Element|!Array<!Element>|!NodeList|!HTMLCollection} elements
  The elements we wish to upgrade.