This page contains some useful links to resources with information about XML and XSLT.

Xerces-C is an excellent XML parser with a C++ API and support for SAX, DOM, XSD and DTDs (to name a few things). It is used internally by gnomon to parse XML and stylesheets (via Xalan).

Xalan-C is an excellent XSLT processor, built on Xerces' SAX parser. It has a C++ API, and full support for the W3C standard, as well as some EXSLT extensions.

XSL transformations (XSLT) standard is the reference point for XSL.

The mulberry XSL list is an excellent place to get your XSLT questions answered.

Robin Cover's XSL page on oasis has a detailed history of XSL and many links to useful resources.