[1] Compared to other XML schema languages RELAX NG is considered relatively simple.
It was defined by a committee specification of the OASIS RELAX NG technical committee in 2001 and 2002, based on Murata Makoto's RELAX and James Clark's TREX,[2][3][4] and also by part two of the international standard ISO/IEC 19757: Document Schema Definition Languages (DSDL).
Since then, however, RELAX NG support has increasingly found its way into XML software, and its acceptance has been aided by its adoption as a primary schema for popular document-centric markup languages such as DocBook, the TEI Guidelines, OpenDocument, and EPUB.
By informal convention, RELAX NG schemas in the regular syntax are typically named with the filename extension ".rng".
Note that Trang is unable to convert the OASIS DITA 1.3 schema to DTDs, failing with messages like: