Henri Sivonen has implemented an online Validation Service for RELAX NG that can validate against standard (Docbook, XHTML. etc.) and user supplied schemas. "The validation service checks whether a given document meets the constraints of the chosen schema(s). Both XML syntax and compact syntax RELAX NG schemas are supported. Also, there is 'experimental' support for standalone (not embedded) Schematron 1.5 schemas. Multiple schemas at a time are supported."