Also note that bullet 5 in the previous section states: 

"If the assistive technology sets focus to a descendant of a container with aria-activedescendant, the user agent MUST change the value of aria-activedescendant to point to that item."

This seems redundant with the bullet in 3.3 Handling focus changes from the assistive technology and is missing the part about setting focus on the ancestor. Can this bullet be removed from 3.2 Controlling focus with aria-activedescendant.

(In reply to comment #1)
> Also note that bullet 5 in the previous section states: 
> 
> "If the assistive technology sets focus to a descendant of a container with
> aria-activedescendant, the user agent MUST change the value of
> aria-activedescendant to point to that item."
> 
> This seems redundant with the bullet in 3.3 Handling focus changes from the
> assistive technology and is missing the part about setting focus on the
> ancestor. Can this bullet be removed from 3.2 Controlling focus with
> aria-activedescendant.

Sure.

(In reply to comment #0)
> 2nd bullet says "Otherwise, if the current element has an ID and an ancestor
> with the aria-activedescendant attribute present, then set DOM focus to that
> ancestor. If successful, then set aria-activedescendant on the ancestor to the
> ID of the desired element." 
> 
> What happens if it is not successful.

Do we need to define that behaviour? I believe on FF focus will go on the nearest focusable parent (up the parent chain).

This is an author error. Inspecting our FF code we do the following:

If an AT requests 'takefocus' on an accessible foo that is nested underneath a containing element with an aria-activedescendent attribute FF will set focus on the containing element if it is focusable. If not the best we can really do is attempt to set focus on foo itself.

Perhaps we could say something like this:

"When it is not possible for the user agent to set DOM focus to the containing element with aria-activedescendent, the user agent MAY attempt to set focus to the child element itself. Note: the inability to set focus to the containing element indicates an author error."

Added suggested text.