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SGML, the Standard Generalized Markup Language, allows allows to produce TeX,PS,HTML,RTF files from a single source file.
SGML is an official International Standard for the electronic interchange of information. HTML is a simple application of SGML. XML, the Extensible Markup Languate is an application profile of a restricted form of SGML. By construction, XML documents are conforming SGML documents.
Additionally to the SGML file example howto.sgml , there is a Document Type Definition DTD, which the SGML file has to conform to. Examples of DTL's are the linuxdoc DTD used in guide.sgml . To produce txt, latex, html, rtf files from the sgml file, run it through a translator.
sgml2txt guide.sgml
sgml2latex guide.sgml
sgml2html guide.sgml
sgml2rtf guide.sgml
sgmlcheck guide.sgml

Simplicity, Clarity, Generality B.W. Kernighan, R. Pike, in "The Practice of Programming".