select 下拉框分組

<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>