@aria-activedescendant is being exposed inconsistently.
WebKit exposes it under AXSelectedRows for some controls like AXOutline (role="tree"), AXList (role="listbox"), and possibly AXMenu (role="menu"), but WebKit doesn't expose @aria-activedescendant at all on some controls, like generic textfields. WebKit does expose it (as the AXSelected option in the AXList) in comboboxes if the author hooks up the AXOwns relationship properly to point to the popup listbox.
However WebKit exposes @aria-activedescendant, it should be more consistent.
Update: aria-activedescendant works in Chrome (not Safari) in combination with a role of textbox. Combined with a role of combobox, aria-activedescendant does not work in Chrome or Safari.
*** This bug has been marked as a duplicate of bug 161734 ***