ISO, W3C, now OASIS...

W3C XML Schema not a 'home run' W3C XML Schema hasn't won the hearts and minds of the entire XML community. It may be a marketing success in terms of promised support from IBM, Microsoft, and Oracle, but you don't have to look far or hard to find complaints. Too complex, too kludgy, too type-centric, too oriented to OOP and relational databases, too quirky, etc.
RELAX appeared from JIS, then to ISO Murata Makoto took his schema ideas to the Japanese national standards body, and RELAX became a JIS technical report. It's currently in ISO balloting for publication as a technical report.
and Schematron Rick Jelliffe developed a schema approach based on XSLT and XPath which took a different route, but had some of the same capabilities (context-sensitivity) as RELAX.
and now TREX goes to OASIS James Clark has built his own schema language, with ideas from RELAX and to some extent from SGML. It's at OASIS now, as a technical committee has just been formed.