[Responder] Jeremy Orlow
Backwards compatibility is central to HTML. If your HTML needs to target specific versions of the spec, then we've failed. Adding the notion of versions to the DOCTYPE will simply confuse authors and provides no real benefit as far as I can tell. If we do for some reason, in the future, do need to break backwards compatibility, we should then consider adding a mechanism for citing versions. Until then, the version number will just be noise/overhead/confusing.