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Chapter 37. Toward XML > Understanding the Relationship Between SGML, XML, and ...

Understanding the Relationship Between SGML, XML, and XHTML

XML is a meta-language, a special language that allows you to completely describe a class of other languages, which in turn describe documents. It's like an island in the sea of SGML, another and more powerful meta-language. XML is defined as a "proper subset" of SGML, which means no pieces are added to SGML to make XML, but pieces are taken away to make the language easier to parse, understand, and use. XHTML is a production of XML, as HTML is a production of SGML.

Because XML is designed to be extensible, the languages created with XML are extensible as well. As in SGML, the language descriptions created by XML are called DTDs, Document Type Definitions. You'll find out much more about them later. But first, take a look at SGML, the mother of XML and its related standards.


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