<selectmenu> is a new experimental form element that takes the place of <select>, with a goal of being a lot more customisable.
The default markup is very similar to a regular <select>:
<selectmenu> <option>Option 1</option> <option>Option 2</option> <option>Option 3</option> </selectmenu>
However, more parts are targetable with [[css]], and even the markup is more customisable. A custom button can be introduced to control the open/closed state.