See single selection example:
https://rawgit.com/w3c/aria-practices/master/examples/listbox/listbox.html
In Chrome, as you arrow through the list, each item says "unselected". Chrome does not set the SELECTED state for these items.
In Firefox, this is not spoken, and the SELECTED state is set for each item.
For containers that support selection but are not in multiple selection mode, we should set SELECTED state unless aria-selected="false".
Comment 1 by aleventhal@chromium.org
, May 17 2017