W3.org:

W3.org has 287 URLs listed in 163 Dmoz categories.

The W3C Markup Validation Service
The W3C Validation Service validates multiple versions of XHTML and HTML, outputting many useful errors and warnings to help users create a standards-compliant web page.
http://validator.w3.org/
Top/Computers/Data_Formats/Markup_Languages/XHTML/Tools

XML Activity Statement
An executive overview of W3C's current and historical work on the Extensible Markup Language (XML).
http://www.w3.org/XML/Activity
Top/Computers/Data_Formats/Markup_Languages/XML

Extensible Markup Language (XML)
Main page for World Wide Web Consortium (W3C) XML activity and information.
http://www.w3.org/XML/
Top/Computers/Data_Formats/Markup_Languages/XML

XML Path Language (XPath) Version 1.0
Official W3C site describing this language for addressing parts of an XML document which is designed to be used by both XSLT and XPointer. (W3C Recommendation 16 November 1999)
http://www.w3.org/TR/xpath
Top/Computers/Data_Formats/Markup_Languages/XML/Addressing_and_Querying/XPath

DOM Level 3 XPath Specification
Provides simple functionalities to access a DOM tree using XPath 1.0. This module builds on top of the Document Object Model Level 3 Core. [W3C Working Draft 31 October 2001]
http://www.w3.org/TR/DOM-Level-3-XPath/
Top/Computers/Data_Formats/Markup_Languages/XML/Addressing_and_Querying/XPath

XML Pointer Language (XPointer)
A specification of constructs that support addressing into the internal structures of XML documents. It provides for specific reference to elements, character strings, and other parts of XML documents, whether or not they bear an explicit ID attribute. (
http://www.w3.org/TR/xptr/
Top/Computers/Data_Formats/Markup_Languages/XML/Addressing_and_Querying/XPointer

W3C: XML Query
Home of the W3C's XML Query working group, whose work includes XQuery and XPath.
http://www.w3.org/XML/Query/
Top/Computers/Data_Formats/Markup_Languages/XML/Addressing_and_Querying/XQuery

Mailing List: www-ql@w3.org
The W3C's public list for discussion of Query Languages, including (but not limited to) discussion on the XML Query project.
http://lists.w3.org/Archives/Public/www-ql/
Top/Computers/Data_Formats/Markup_Languages/XML/Addressing_and_Querying/XQuery/FAQs,_Help_and_Tutorials

XQuery 1.0 and XPath 2.0 Data Model
This document defines the W3C XQuery 1.0 and XPath 2.0 Data Model, which is the data model of at least XSLT and XQuery, and any other specifications that reference it. This data model is based on the data models of XPath and XML Query Data Model and repl
http://www.w3.org/TR/xpath-datamodel/
Top/Computers/Data_Formats/Markup_Languages/XML/Addressing_and_Querying/XQuery/Standards_Documents