code: function applyTemplate(el, tpl) {
          var lightFrag = void 0,
              handleContentNode = hasContent(tpl);
  
          if (handleContentNode) {
              var newCebContentId = 'ceb-content-' + counter++;
              lightFrag = cleanOldContentNode(el);
  
              tpl = replaceContent(tpl, newCebContentId);
  
              el.setAttribute(OLD_CONTENT_ID_ATTR_NAME, newCebContentId);
          }
  
          el.innerHTML = tpl;
  
          if (handleContentNode && lightFrag) {
              el.lightDOM.appendChild(lightFrag);
          }
      }
docs: Apply the template to the element.
  @param {!HTMLElement} el the custom element
  @param {!string} tpl the template