code: public Content getHeader(String header) {
          HtmlTree bodyTree = getBody(true, getWindowTitle(header));
          HtmlTree htmlTree = (configuration.allowTag(HtmlTag.HEADER))
                  ? HtmlTree.HEADER()
                  : bodyTree;
          addTop(htmlTree);
          addNavLinks(true, htmlTree);
          if (configuration.allowTag(HtmlTag.HEADER)) {
              bodyTree.addContent(htmlTree);
          }
          Content h1Content = new StringContent(header);
          Content heading = HtmlTree.HEADING(HtmlConstants.TITLE_HEADING, true,
                  HtmlStyle.title, h1Content);
          Content div = HtmlTree.DIV(HtmlStyle.header, heading);
          if (configuration.allowTag(HtmlTag.MAIN)) {
              mainTree.addContent(div);
          } else {
              bodyTree.addContent(div);
          }
          return bodyTree;
      }
docs: Get the given header.
  
  @param header the header to write
  @return the body content tree