Extensible Markup Language: XML

XML is a data format for structured document interchange on the Web. The current W3C Recommendations are XML 1.0, Feb '98.

XML is designed 'to make it easy and straightforward to use SGML on the Web: easy to define document types, easy to author and manage SGML-defined documents, and easy to transmit and share them across the Web.'

It defines `an extremely simple dialect of SGML which is completely described in the XML Specification. The goal is to enable generic SGML to be served, received, and processed on the Web in the way that is now possible with HTML.'

`For this reason, XML has been designed for ease of implementation, and for interoperability with both SGML and HTML' [quotes from the XML spec].

Main differences between XML and SGML:

• no tag minimisation (many 'NO' in the SGML declaration)
• it is also different from the RCS (unlimited quantities, capacities; case sensitive names which can use Unicode characters)
• 'and' connector not allowed
• element inclusion and exclusions not allowed
• content models for mixed content have a restricted form (repeatable "or" group only)
• INCLUDE/IGNORE marked sections are not allowed in the document instance
• EMPTY elements are closed with />, e.g. <anchor id="x.2.3"/>

Also important:

• valid XML document: DTD + document instance
• well-formed XML document: document instance only

Related recommendations:

• XSL: Extensible Stylesheet Language
• XLL: XML Linking Languages