html中select optgroup标签使用介绍

<select name="bid" id="brand_3">
<option value="">请选择品牌</option>
<optgroup label="A"></optgroup>
<option value="693" custom="693">A AC宝马</option>
<option value="62" custom="62">A 阿斯顿马丁</option>
<option value="1" custom="1">A 奥迪</option>
<optgroup label="B"></optgroup>
<option value="723" custom="723">B 巴博斯</option>
<option value="44" custom="44">B 保时捷</option>
<option value="582" custom="582">B 宝骏</option>
<option value="20" custom="20">B 宝马</option>
<option value="593" custom="593">B 北京汽车</option>
<option value="643" custom="643">B 北汽威旺</option>
<option value="122" custom="122">B 北汽制造</option>
<option value="4" custom="4">B 奔驰</option>
</select>