The W3C Specifications
W3C releases the specifications for both XML and XSL. These specifications are not called standards because by international agreement, standards are created only by government-approved bodies. Instead, the W3C starts by releasing the requirements for a new specification. The requirements are goals, and list a sort of preview of what the specification will be all about, but the specification isn't written at that point. Next the W3C releases specifications first as working drafts, which anyone may comment on, then as candidate recommendations, which are still subject to review, and then finally as recommendations, which are final.
The following list includes the XSLT-related W3C specifications and where you can find them:
The complete XSL candidate recommendation. This is the big document that specifies all there is to XSL.
The XSL Transformations 1.0 recommendation. XSLT's function is to transform the contents of XML documents into other documents, and it's what's made XSL so popular.
The XSLT 1.1 working draft. This is the proposed upgrade to XSLT 1.0, adding an easy extension mechanism and support for the W3C XML Base specification (you can find more on XML Base at http://www.w3.org/TR/xmlbase/).
The XSLT 2.0 requirements. W3C has released the set of goals for XSLT 2.0, including more support for XML schemas.
The XPath 1.0 specification. You use XPath to locate and point to specific sections and elements in XML documents so that you can work with them.
The XPath 2.0 requirements. XPath is being updated to offer more support for XSLT 2.0.