code: public final T addCssClass(String value) {
      if (StringUtils.isNotEmpty(value)) {
        return setCssClass(StringUtils.isNotEmpty(getCssClass()) ? getCssClass() + " " + value : value);
      }
      else {
        return (T)this;
      }
    }
docs: Html "class" attribute. Adds a single, space-separated value while preserving existing ones.
  @param value Value of attribute
  @return Self reference