Although the Extensible Markup Language (XML) itself has a very short specification for such an important format, there are a lot of specifications in and around XML that are considered very important.

Overview of XML Components

XML Base

Override the default URI of a document or any part of a document starting at a given element;

Stylesheets in XML

Associate an XSLT transformation with an XML document, for example so that a Web browser will format it;

XLink

A vocabulary for hypertext in XML;

xml:id

Identify an XML attribute or element as containing a name that can be used as a unique identifier within a document;

XInclude

Include all or part of other text or XML documents, or duplicate part of the current XML document;

XPointer

This is a framework for different ways to point into XML documents, and is used by XLink;

XForms

A more powerful cousin to HTML forms;

XML Events, XHTML Modularization

Specifications primarily relating to the use of XML in Web browsers or other DOM-based systems;

XML Fragments

Listed here only for completeness; this specification is not in widespread use.

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Contact: Liam R. E. Quin <liam@w3.org>