select optgroup

<select name="test">
    <optgroup label="a">
        <option>a1</option>
        <option>a2</option>
        <option>a3</option>
    </optgroup>
    <optgroup label="b">
        <option>b1</option>
        <option>b2</option>
        <option>b3</option>
    </optgroup>
</select>