The big problem is that to enable full usability of both standards the framework needs to handle everything from both standards and convert the data back and forth and even try to emulate the different gaps that will appear because of the standards different feature sets.
Maybe it would be possible to make each theme have a _config.php file for telling the engine which standard it uses. This would then trigger convertions between the two standards, for example between HTML 5's different section elements and XHTML 2's role attribute.
But then there is a new question: Should the new ways of doing things be used even when it requires more parsing? It would probably be harder to do, but after all, it's the new and probably recommended way.