Would it be possible to have a generic browser that isnt controlled by any vendor, but perhaps contributed to by all of them, which could be used as a dev standard? It doesn't even have to be available to users, just devs. Vendors could still develop their browsers any way they want, but when output diverges from this kit, they would at least know it. As a dev you could use this as your test kit and know that if it works here, it works everywhere.
I could see problems with this being backwards compatible, but at the point where the major vendors sign on, all future releases would compatible (and eventually all releases as older versions are retired).
I suppose that perhaps though this is already accomplished by Chrome, but it seems less than ideal for many reasons.