This document is both a learning experience for me and hopefully a way for others to learn about Rule-based validation - Schematron, ISO/IEC 19757-3:2006(E). That is it's full ISO title, it's the third part of Information technology - Document Schema Definition Languages(DSDL). DSDL brings together various validation tasks to form the single framework enabling them to work together to produce a single set of validation results. Schematron is one of those parts.
The title arises simply because of my starting point. It is also an experiment since it is written using the Docbook Relax ng schema and stylesheets which are moderately new, and impressive!
Schematron provides an elegant and thorough way of validating an XML document for conformance to a set of specified constraints expressed in XML. In plain English Schematron goes further, and in different directions to schema validation using a conventional DTD or schema (relax ng or W3C schema).
I'm grateful to the contributors to the Schematron mailing list for their constructive feedback.