[Responder] Tab Atkins Jr.
Robin Berjon addressed the issue of a version indicator eloquently in his series on "XML Bad Practices", based on his experiences developing the SVG specification: http://berjon.com/blog/2009/12/xmlbp-naive-versioning.html. If content is expected to be rejected across version boundaries, then "versions" are actually different languages entirely. Pretending that they're just different versions helps no one, and so a versioning indicator is not needed. Luckily, HTML has never and likely will never fall into this category - the next category is the relevant one here. If content is expected to be accepted across version boundaries, then good "versioning" behavior must be relatively complex and subtle to correctly handle unknown (read: newly introduced) content. The versioning indicator itself doesn't help in any way with this - what happens if the author says the content is version 2, but uses a construct from version 1 that's obsolete in version 2, and a construct from version 3 that hadn't been thought of at the time version 2 was created? Proper error-handling (for the version1 construct) and forward-compatible design (for the version3 construct) needs to be designed into the language in the first place, and an additional version indicator provides precisely zero help here. Thus, a versioning indicator is never needed. As this appears to be the primary justification for this change proposal, then, I strongly object to changing the language in this way.