htmlのselect optgroupタグの使い方の紹介

<select name="bid" id="brand_3">
<option value="">Please select brand</option>
<optgroup label="A"></optgroup>
<option value="693" custom="693">A AC BMW</option>
<option value="62" custom="62">A Aston Martin</option>
<option value="1" custom="1">A Audi</option>
<optgroup label="B"></optgroup>
<option value="723" custom="723">B Babs</option>
<option value="44" custom="44">B Porsche</option>
<option value="582" custom="582">B Baojun</option>
<option value="20" custom="20">B BMW</option>
<option value="593" custom="593">B Beijing Auto</option>
<option value="643" custom="643">B Beiqi Weiwang</option>
<option value="122" custom="122">B BAIC Manufacturing</option>
<option value="4" custom="4">B Mercedes-Benz</option>
</select>