The OPTGROUP element defines a group of choices within a SELECT menu.
The OPTGROUP element defines a group of choices within a SELECT menu. OPTGROUP must contain one or more OPTION elements to define the actual choice
Example:
<select>
<optgroup label="Swedish Cars">
<option value="volvo">Volvo</option>
<option value="saab">Saab</option>
</optgroup>
<optgroup label="German Cars">
<option value="mercedes">Mercedes</option>
<option value="audi">Audi</option>
</optgroup>
</select>
sources: http://htmlhelp.com/reference/html40/forms/optgroup.html, http://www.w3schools.com/tags/tag_optgroup.asp
