code: function findContentNode(el) {
          if (!el) {
              return;
          }
          var oldCebContentId = el.getAttribute(OLD_CONTENT_ID_ATTR_NAME);
          if (oldCebContentId) {
              return findContentNode(el.querySelector('[' + oldCebContentId + ']')) || el;
          }
          return el;
      }
docs: Try to find a light DOM node
  @param {!HTMLElement} el the custom element
  @returns {HTMLElement} the light DOM node if found otherwise it's the given HTML Element