Except (surprise!) in other W3C XML specs

Escaping Doing XHTML Modularization using DTDs was difficult enough, but WXS was just too much. So the W3C switched to RELAX NG, an alternative specification from OASIS and ISO. The W3C uses RELAX NG for its work and generates both DTDs and W3C XML Schema from the RELAX NG foundation
XHTML, SVG, XML Signature, RDF RELAX NG turns out to have a lot more expressive power than W3C XML Schema for a wide variety of tasks, and a lot less cruft.
Combinations Creating combinations of W3C specs using RELAX NG is much simpler than it was in DTDs or W3C XML Schema. Integration of things like the "XHTML+SVG+MathML Profile" is just plain natural.
For more information, see: http://lists.w3.org/Archives/Public/www-svg/2003May/0069.html
Web Services an exception: Though WSDL is making room for RELAX NG, most tools and specs in this space are WXS-centric.