Relax NG is a schema language for XML
Summary
RELAX NG is a simple, easy to learn XML schema language offering both XML and compact syntax and compatibility with namespaces and the XML information set. The RELAX NG page on relaxng.org also acts as a rich resource hub, listing tutorials, specifications, books, validators, converters, editors, and schemas from various technologies, maintained by MURATA Makoto. Note that the page history shows last updated in 2014, so users should verify current status for newer developments.