This article describes the aria-activedescendant property.
The aria-activedescendant attribute contains the ID of the currently active child object that is part of a composite widget within the Document Object Model. It makes do with the overhead of having all or more than one child focusable. As the name specifies, it helps in managing the current active child of the composite widget.
The user agent, which is any software that retrieves, renders and facilitates end user interaction with Web content, uses the aria-activedescendant property to inform the assistive technology about the active child which has focus. This active child using the aria-activedescendant property must always be visible on screen and must be a descendant of the container in the Document Object Model.
TBD: Add support information for common UA and AT product combinations