An <optgroup> can be used inside of a <select> to group <option>s. They can contain zero or more <option> elements, and may not be nested.
<select id="dino-select">
<optgroup label="Theropods">
<option>Tyrannosaurus</option>
<option>Velociraptor</option>
<option>Deinonychus</option>
</optgroup>
<optgroup label="Sauropods">
<option>Diplodocus</option>
<option>Saltasaurus</option>
<option>Apatosaurus</option>
</optgroup>
</select>