Suddenly? No. Most of the web is still a non-wellformed mixture of HTML3, HTML4, and imaginary tags made up by specific browsers. However, current browsers do choke on non-wellformed markup if it is served with a content-type of text/xml, and that's a first step. As things like XSLT and RDF start to catch on, sites that want to harness the value of those things will have to be redone in wellformed XML, and that's that. (They won't necessarily have to provide and validate against Schemata, but we have to start someplace.)
Incidentally, if CGI.pm is now improved to the point of being capable of producing anything that remotely resembles XHTML, maybe I should have another look at it; I've been avoiding it because of two things, and one was the execrable state of its output. If that has been shaped up, maybe the other thing (the tendency to obfuscate the Perl code) has been improved too, since I looked at it (which has been a bit), and I should have a second look.