third_party/aria-practices/src/content/patterns/menu-button/menu-button-pattern.html
A menu button is a button that opens a menu as described in the Menu and Menubar Pattern. It is often styled as a typical push button with a downward pointing arrow or triangle to hint that activating the button will display a menu.
button element that opens a menu of actions or commands where focus in the menu is managed using element.focus().a element that opens a menu of items that behave as links.With focus on the button:
The keyboard behaviors needed after the menu is open are described in the Menu and Menubar Pattern.
button has aria-haspopup set to either menu or true.button has aria-expanded set to true. When the menu is hidden, aria-expanded is set to false.button has a value specified for aria-controls that refers to the element with role menu.