code: @Override
      public void setHTML(String html) {
          if (html == null) {
              if (header != null) flow.remove(header);
          } else {
              if (header == null) {
                  header = new Heading(1);
                  flow.add(header);
              }
              header.getElement().setInnerHTML(html);
          }
      }
docs: Sets the value of the header(Hn) element