Simon points out that RELAX NG [is] now an ISO standard. I'm definitely going to mine RELAX NG for concepts for something else I'm working on. Now that I know they have a compact syntax, I'm much more willing to explore it. My eyes get blurry looking at too much XML.
This announcement at xmlhack has a bunch of useful links at the bottom, as does page 3 of the "compact syntax" article linked above. Also see http://relaxng.org/
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