Making it a live region makes sure screen readers speak the output as soon as it happens without the user having to review every bit of it manually. This can be done by adding aria-live="polite" to the top most parent element of what contains the chat output. So if there's a div around the elements containing all the conversation text, use that, or the body element of that HTML output widget.
Created attachment 631342 [details] [diff] [review] Patch Marco, if I understood what you want correctly, this trivial patch should do it. Note: we received a very similar request for Instantbird at https://bugzilla.instantbird.org/show_bug.cgi?id=1262 and an older request at https://bugzilla.instantbird.org/show_bug.cgi?id=103 which we understood at the time as wanting the aria attribute on each message, rather than the whole message area. That was more difficult to enforce (as message themes can be customized) and regressed since the fix.
Comment on attachment 631342 [details] [diff] [review] Patch Yes, this looks correct!
Comment on attachment 631342 [details] [diff] [review] Patch [Approval Request Comment] I think we want accessibility fixes of the chat feature on aurora for Tb15.
Comment on attachment 631342 [details] [diff] [review] Patch Yes definitely.