aria-expanded
Widget attributes might be mapped by a user agent to platform accessibility API state, for access by assistive technologies, or they might be accessed directly from the DOM. User agents MUST provide a way for assistive technologies to be notified when states change, either through DOM attribute change events or platform accessibility API events.
Indicates whether the element, or another grouping element it controls, is currently expanded or collapsed.
For example, this indicates whether a portion of a tree is expanded or collapsed. In other instances, this may be applied to page sections to mark expandable and collapsible regions that are flexible for managing content density. Simplifying the user interface by collapsing sections may improve usability for all, including those with cognitive or developmental disabilities.
If the element with the aria-expanded attribute controls the expansion of another grouping container that is not 'owned by' the element, the author SHOULD reference the container by using the aria-controls attribute.