XML as a subset of SGML

XML guidelines are set forth by the XML Working Group of the World Wide Web Consortium (W3C) as reflected in the XML Version 1.0 Specification accepted by the W3C as a Recommendation on Feb 10, 1998.  Adoption of XML requires that it support all platforms, many applications, and the legacy of SGML (Standard Generalized Markup Language). XML is a subset or restricted form of SGML (ISO 8879). The goal of XML is to enable generic SGML to be served, received, and processed on the Web in the way that is now possible with HTML. XML has been designed for ease of implementation and for interoperability with both SGML and HTML.

XML customizes SGML in a number of significant ways:

• a specific choice of syntax characters was made so that everyone using XML will use the same concrete syntax. For example all opening tags must begin with < and end with >.
• a new empty-element tag may be used to indicate that this is an empty element and that an end tag is not expected. This new empty-element tag is like a start tag with a slash character just before the closing greater-than angle bracket.
• tag omission is not allowed as it is in SGML. This means that each non-empty element will have a both a start tag and an end tag.
• XML does not require that a Document Type Definition (DTD) be present.

XML's job is to compose and deliver content as efficiently as possible. Instead of obscure HTML markup like <P> and <TR>, XML is spelled out (e.g., <PARAGRAPH> and <TABLEROW>). XML seeks to make its markup clear, as opposed to short.

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