Although RELAX NG started as a standalone project under the auspices of the Organization for the Advancement of Structured Information Standards (OASIS), RELAX NG is now being standardized at ISO (ISO/IEC JTC1 SC34 WG1, to be precise) as a part of a multipart standard named DSDL (see http://dsdl.org).

DSDL (Document Schema Definition Languages) recognizes that the validation of XML documents is a subject too wide and complex to be covered by a single language. It also acknowledges that the industry needs a set of simple and dedicated languages to perform different validation tasks—as well as a framework in which these languages may be used together.

Validating (or schematizing) XML documents may involve:

Throughout this book, you've seen that RELAX NG is simple and efficient because it stays focused on solving one—and only one—problem. There are huge gaps that can't be covered by RELAX NG. For instance, if an XML vocabulary includes mixed-content models, you can't restrict the content of your documents to ASCII; neither can you define that the content of your modeling element must be spell-checked. The goal of DSDL is to provide a means to fill out these gaps and to cover the whole domain of document validation.

DSDL can be seen as a framework and set of languages that checks the quality of XML documents, a crucial issue for any XML based application. Recent works such as the presentation given by Simon Riggs at XML Europe 2003 or the work of Isabelle Boydens (Informatique, normes et temps, Bruxelles, Éditions E. Bruylant, 1999.) about the quality of large databases have shown that about 10% of XML documents (or data records) contain at least one error. This level of quality is unacceptable for many applications; and so DSDL can be an absolutely indispensable technology for many XML applications.