The default behavior of the <selectmenu> control mimics the behavior of the <select> command. One way to style the control to match you requires using the CSS: ::part() pseudo-element to select the different parts within the control’s anatomy that you wish to style. ::part() can be used to style the <button>, <selected-value>, and <listbox> parts of the control. You can also customize the control much more by providing your own markup to replace the default one, and extend or re-order the parts.
