code: public void addContent(Content tagContent) {
          if (tagContent instanceof ContentBuilder) {
              for (Content content: ((ContentBuilder)tagContent).contents) {
                  addContent(content);
              }
          }
          else if (tagContent == HtmlTree.EMPTY || tagContent.isValid()) {
              if (content.isEmpty())
                  content = new ArrayList<Content>();
              content.add(tagContent);
          }
      }
docs: Adds content for the HTML tag.
  
  @param tagContent tag content to be added