<select multiple>
<optgroup label="First Group">
<option value="1.1">Element 1.1</option>
<option value="1.2">Element 1.2</option>
<option value="1.3">Element 1.3</option>
<option value="1.4">Element 1.4</option>
</optgroup>
<optgroup label="Second Group">
<option value="2.1">Element 2.1</option>
<option value="2.2">Element 2.2</option>
<option value="2.3">Element 2.3</option>
<option value="2.4">Element 2.4</option>
</optgroup>
<optgroup label="Third Group">
<option value="3.1">Element 3.1</option>
<option value="3.2">Element 3.2</option>
<option value="3.3">Element 3.3</option>
<option value="3.4">Element 3.4</option>
</optgroup>
</select>