HTML optgroup label 属性

<select>
       
      <optgroup label="Swedish Cars">
         
      <option value="volvo">Volvo</option>
         
      <option value="saab">Saab</option>
       
      </optgroup>
       
      <optgroup label="German Cars">
         
      <option value="mercedes">Mercedes</option>
         
      <option value="audi">Audi</option>
       
      </optgroup>
 </select>