code: public HTMLForm addClass(String clazz) {
          String currentClass = this.getProperty("class");
          if (currentClass != null) {
              currentClass += " " + clazz;
          } else {
              currentClass = clazz;
          }
          this.setProperty("class", currentClass);
  
          return this;
      }
docs: Adds the given class to the {@code class} property of this form.
  
  @param clazz HTML class to add
  @return This form