code: public void setHtml(String html) {
          this.html = html;
  
          if (widget != null) {
              if (widget.isAttached()) {
                  tooltipElement.find("span")
                          .html(html != null ? html : "");
              } else {
                  widget.addAttachHandler(event ->
                          tooltipElement.find("span")
                                  .html(html != null ? html : ""));
              }
          } else {
              GWT.log("Please initialize the Target widget.", new IllegalStateException());
          }
      }
docs: Set the html as value inside the tooltip.