RelaxNG versus W3 XSD
xmlhack: Expressing mixed content in RELAX NG versus WXS http://xmlhack.com/read.php?item=1880
This is a nice reference. We are experimenting with expressing the blocklayout language in RNG, exactly because the XSD stuff is just plain unreadable. The RelaxMeter test for ambiguity is especially interesting for checking a solution specified in RNG.