Resin's XML and XSL support follows the W3C specification.
Resin's XML parser uses the JAXP API.
XSL's power comes from the XML path language, a pattern matching language for selecting nodes in an XML tree. The most useful subset is covered here. For the full description, see the XPath 1.0 spec.
Reference guide to the XSL tags. The most useful subset is covered here. For the full description, see the XSLT 1.0 spec.
For those put off by XSL's verbosity, Resin offers StyleScript. StyleScript retains the full capabilities of XSLT. It simplifies HTML generation by focusing on uninterpreted text, so '<' produces 4 characters. StyleScript also provides some syntactic sugar.
If you're unfamiliar with XSL, the introduction gives a brief overview of creating web pages with XTP and XSL.