On XHTML, I think there was room for both HTML and a proper XHTML that barks on errors. If you're a human typing HTML or using a language where you build your HTML by concatenation like early PHP, sure it makes sense to allow loosey goosey HTML but if you're using any sort of simple DOM builder which should preclude you from the possibility of outputting invalid HTML, strict XHTML makes a lot more sense.

Honestly I'm disappointed the promised XHTML5 never materialized along side HTML5. I guess it just lost steam.