The fundamental problem with a version # in a document is that it doesn’t provide for a given document to be valid under more than one version. What we really need is to be able to indicate a “space of versions” that a given document is valid under…
For years now, I’ve been asserting that the best way to do this with XML is to re-use the namespace name for compatible versions and to ignore things it doesn’t understand. That is, there won’t be a change in the namespace name for compatible changes. The space of versions is the single namespace name. The consumer won’t need to distinguish between versions and it will ignore any extra items it doesn’t understand.
Even the TAG
its position on versioning.
long as such documents are likely to have the same or
sufficiently compatible meanings per the different versions,
then it may be better to omit any indication of version in the
instance, and leave it to the receiving software to decide
whether the document can be processed.
So basically, like Mark
Baker (and potentially the TAG) say,
serving content as
already the sort of “version” indicator we want,
since HTML 5 is backwards compatible with the public web. In the
fulfills the same role—which is why the XHTML 2 folks
should really pick a new namespace URI. Hopefully, they’ll
do so as a part of their rebranding