RELAX NG
In computing, RELAX NG (REgular LAnguage for XML Next Generation) is a schema language for XML - a RELAX NG schema specifies a pattern for the structure and content of an XML document. A RELAX NG schema is itself an XML document but RELAX NG also offers a popular compact, non-XML syntax. Compared to other XML schema languages RELAX NG is considered relatively simple.