I have recently recommended to a large publishing client that they adopt RELAX NG as the basis of the formal definitions of their content, in preference to W3C XML Schema Definition Language (WXS). There are lots of individual bits of information on why RELAX NG should be preferred all over the web. Here is an attempt to condense some of the key information into 10 points

1. A better spec means better interoperability
2. Availability of a compact syntax
3. The specification is a stable ISO standard
4. No PSVI
5. No content defaulting
6. Better datatyping support
7. More sophisticated modelling
8. More sophisticated grammatical validation
9. Instances have no dependency
10. Growing consensus

now hopefully kml will get cured of the XSD disease