there are plenty of RELAX NG validators available: jing, rnv, MSV, one in Mono libraries, …
RELAX NG + Schematron validator should be used
relames (Schematron plugin for MSV)
Relaxed—on-line DocBook V5.0 validator available at http://relaxed.vse.cz/docbookvalidator/
oXygen
schema associations
RELAX NG idea is to separate schema from document instance
schema to be used is provided externally—manually or using configuration file
some applications support proprietary PIs for associating schema