Standard Generalized Markup Language (SGML) is the parent language of HTML. SGML grew out of an earlier language called Generalized Markup Language (GML) developed around 1970 by Charles Goldfarb and others and an IBM version called Document Composition Facility Generalized Markup Language (DCF GML) (Bryan 1988).
The development of GML and its successors, Document Composition Facility Generalized Markup Language (DCF GML) and SGML, was guided by two principles: textual markup should describe logical structure instead of physical appearance, and textual markup should be unambiguous and easily understood by either a program or a person (Bryan 1988).
After several years of further development, Standard Generalized Markup Language (SGML) was issued in 1986 as Standard 8879 by the ISO (Bryan 1988). As a result of the attempt to make SGML generally suited to almost any markup text, what emerged is "a metalanguage for generating descriptive markup languages" (Coombs, Renear, and DeRose 1987) rather than any specific markup language.
By 1993, SGML had become very popular:
The rapid spread of SGML exceeds anything we anticipated in 1987 and does not appear to be slowing. It is far from a perfect markup system, but it has been very effective in focusing attention on document structure rather than transient formatting and in facilitating document interchange (Bryan 1988)
There are indeed some problems with SGML. Page fidelity is not preserved; that is, there is no facility for specifying the exact pagination and textual look of a document (DeRose 1993). Complicated structures such as tables and equations can be described with SGML, but the descriptions are cumbersome (DeRose 1993). Finally, SGML has been criticized as too complicated, but tools can be developed to make the process of document composition and management easier (Coombs, Renear, and DeRose 1987). In addition, for a particular application, languages generated by SGML can be used instead of the full range of capabilities of SGML. This is exactly what happened with the World Wide Web and HTML.
Overall, SGML was a crucial step in the development of the Web. The fact that it or its precursors had been aroung in some form or another for twenty years prior to the Web, combined with the international standard, made SGML an excellent choice for generating a specific language, HTML, for use on the Web.