The Road to XHTML 2.0: MIME Types (O'Reilly)
Mark Pilgrim
writes about the migration to XHMTL 2.0 on O'Reilly.
"
Now here's a dirty little secret: browsers aren't actually treating your XHTML as XML. Your validated, correctly DOCTYPE'd, completely standards compliant XHTML markup is being treated as if it were still HTML with a few weird slashes in places they don't belong."
Why? The answer is MIME types.