The current UA draft doesn t seem to cover spawned browser windows entirely new copies of the browser that can currently be generated without the user s awareness.
Spawned windows present a number of accessibility issues.
For instance, I can t even describe the confusion I recently witnessed when a server error message got routed to an invisible browser window that didn t have the focus. The potential for spawned browsers housing nasty applets that a blind user has no way to know about is another problematic issue. Every open copy of a browser should have a separate, accessible identifier.