code: public static String document(ContainerTag htmlTag) {
          if (htmlTag.getTagName().equals("html")) {
              return document().render() + htmlTag.render();
          }
          throw new IllegalArgumentException("Only HTML-tag can follow document declaration");
      }
docs: Return a complete html document string
  
  @param htmlTag the html content of a website
  @return document declaration and rendered html content