As I recall even in draft standards you could mix XHTML 1.0 namespaces and XHTML 2.0 namespaces to use "backwards compatible markup" where you needed to. It was

As I recall even in draft standards you could mix XHTML 1.0 namespaces and XHTML 2.0 namespaces to use "backwards compatible markup" where you needed to. It wasn't pretty as most things involving XML and lots of mixed namespaces isn't, but it was possible. We still don't know if it would have gotten more backwards compatible had browsers tried to actually implement it.