The problem with this approach is the same problem people serving XHTML as application/xhtml+xml ran into: You don't control your page any more these days. If

The problem with this approach is the same problem people serving XHTML as application/xhtml+xml ran into: You don't control your page any more these days. If you allow people to leave comments on your page or if you are serving ads, you lose a bit of control in what gets put on a particular page of yours. In case of comments you could try and sanitize them, but with ads that's hardly possible.