See this post from Anne Kesteren. Marvel at the ridiculousness of the proposal.
However, solving the generic eXtensibility problem for HTML is hard and it’s not that clear what the best approach would be.Sorry to be flippant, but extensible markup is a solved problem. Only the optimisations for legacy/liberal support in text/html+html5 are in conflict with those of open extensibility (which inherently explodes the number of possible markup combinations that have to be considered).
Personally I don’t see the creation of a hobbled form of SVG etc. as a particularly convincing step.
Possibly the only sane approach would be to leave that version without extensibility, and encourage anyone that needs it to use XHTML5. Horses are a perfectly good means of transport, but you can’t really bolt wings to them and expect them to fly.
He’s right: extensible markup is a solved problem. You’d be lying if you said XML wasn’t in widespread use. The problem is that for all the smart people in the web development world, they can’t see that if you can’t have the moon on a stick without breaking some eggs, you need to get out of the kitchen. Piling all of these things into the HTML serialisation is just plain nuts. Just as with Microformats, it’s exactly what XML was designed for. The only small comfort is that people at the W3 recognise this. Money quote from timbl (dude, he invented the friggin WWW, have you heard of it?):
… The idea of using SVG without XML is horrifying.