[ACTION-925] When to fire system alert messages?

We discussed ACTION-925 [1] in the F2F last week and could not come to a
resolution. I got an action item to summarize the issue.

Rich opened this action item because, in the UAIG, we specify that UAs
should fire a system alert for roles alert and alertdialog. But we never
specify when they should fire this alert.

We do mention system alerts at the end of section 5.8.2 Changes to document
content or node visibility [2] but only in the case of MSAA where there is
a change in the visibility state of an object with an ancestor of
role="alert".

Cynthia thinks browser support is not consistent.

James has a concern with browser performance. From the meeting notes, here
are the points:
* it's an expensive DOM action to monitor
* if you have a live region and something is inserted into it, the code is
an expensive computational effort
* anytime the browser needs to expose anything to the AAPI, there is a
performance hit
* maybe we need something in the authoring guide - do we want to encourage
authors to set up live regions and make changes to those regions, or if
they create live regions on the fly should they be mapped automatically?

James, please add to this or correct anything that I got wrong. The minutes
on this topic were a little sparse.

We want to discuss this at tomorrow's UAIG meeting - 12 noon ET/9 AM PT.
David, will you be there? James, can you still come?

[1] http://www.w3.org/WAI/PF/Group/track/actions/925
[2] http://www.w3.org/WAI/PF/aria-implementation/#mapping_events_visibility

Andi