code: protected static HeadElement getHead() {
          if (head == null) {
              final Element element = Document.get().getElementsByTagName("head").getItem(0);
              assert element != null : "HTML Head element required";
              final HeadElement head = HeadElement.as(element);
              AbstractInjector.head = head;
          }
          return AbstractInjector.head;
      }
docs: Gets the document header.
  
  @return the document header