third_party/aria-practices/src/content/patterns/disclosure/disclosure-pattern.html
A disclosure is a widget that enables content to be either collapsed (hidden) or expanded (visible). It has two elements: a disclosure button and a section of content whose visibility is controlled by the button. When the controlled content is hidden, the button is often styled as a typical push button with a right-pointing arrow or triangle to hint that activating the button will display additional content. When the content is visible, the arrow or triangle typically points down.
When the disclosure control has focus:
button has aria-expanded set to true. When the content area is hidden, it is set to false.button has a value specified for aria-controls that refers to the element that contains all the content that is shown or hidden.