Nested OPTGROUP
HTML
<select>
<optgroup label="Level One">
<option> A.1 </option>
<optgroup label=" Level Two">
<option> A.B.1 </option>
<optgroup label=" Level Three">
<option> A.B.C.1 </option>
</optgroup>
</optgroup>
<option> A.2 </option>
</optgroup>
</select>