<form>
<select>
<optgroup label="Grouping 1">
<option value="0">Item 1</option>
<option value="1">Item 2</option>
<option value="2">Item 3</option>
<optgroup label="Grouping 2">
<option value="3">Item 4</option>
<option value="4">Item 5</option>
<option value="5">Item 6</option>
</select>
</form>