The HTML <optgroup> tag is used for grouping related options within your select list. This makes it easier for users to comprehend their choices when looking at a large list.
Difference between HTML and XHTML:
NONE
Example:
<select>
<optgroup label="India">
<option value ="mumbai">Mumbai</option>
<option value ="delhi">Delhi</option>
</optgroup>
<optgroup label="USA">
<option value ="maryland">Marland</option>
<option value ="newyork">New York</option>
</optgroup>
</select>