I have been working in XML filed for a long time, havn't worked with DTD and XSD. I haven't used Relax NG schema yet.
I was asked by someone to take a look at Relax NG. It seems it is getting more popular (although not as popular as XSD). Here is a few high profile use case:
Relax NG is establishing a presence as a language for pivot schemas, that is, a canonical schema from which schemas in other formats are generated by automatic conversion. Examples:
- DocBook uses Relax NG as the normative schema, and uses it to generate XSD and DTD schemas.
- OpenOffice http://www.oasis-open.org/committees/download.php/6037/office-spec-1.0-cd-1.pdf
- The Open Document Format (odt)
- Office Open XML (OOXML) in 2008 released a standard with normative XSD and informative Relax NG. The OOXML Technical Group is currently debating whether Relax NG should be the normative schema for the next release.
- DITA NG provides a Relax NG implementation of DITA 1.2
- W3C uses (non-normative) Relax NG schemas for some of their own specifications, such as XML Security.