It looks like Safari used to expose this via AXARIAControls, which is why we originally supported this that way. However, they now adhere to AAM along with Chrome, which says to use AXLinkedUIElements. VoiceOver won't announce accs exposed via AXARIAControls any more :( I've verified this manually in addition to running the automated tests that this patch updates Differential Revision: https://phabricator.services.mozilla.com/D311962