※<optgroup label=""></optgroup>
(終了タグあり)
26サンプルコード
<select name="sample">
<optgroup label="a">
<option value="a1" label="a1">a1</option>
<option value="a2" label="a2">a2</option>
</optgroup>
<optgroup label="b">
<option value="b1" label="b1">b1</option>
<option value="b2" label="b2">b2</option>
</optgroup>
</select>