Why do you think AXR would have the same interpretation across all browsers? Dont you think somone cough will make their own implementation that fucks up the rendering/layouting just for the heck of it? cough
As long as the reference implementation gets a decent head start, any re-implementers will practically be forced to maintain complete compatibility with it. This model works quite well for programming languages.
Why would anyone do that? That's WAY too much work just for fucking something up just for the heck of it.