Generating HTML documentation from DTD
I consider myself fairly fluent in XML technologies, but there is at least one technology that I have never bothered to fully learn, nor that I am planning to ever fully learn, and this would be XML DTDs.
Some people would like DTDs to disappear completely off the face of XML, and while I wouldn’t disagree with them, I still have to live in a world where some markup languages are formalized using DTDs, and even more importantly, where the greatest part of the developed markup language productions is checked using DTD (typically with the W3C Markup Validator).