XML in a Nutshell, 3rd Edition

SGML’s Legacy

XML is a simplified form of the Standardized General Markup Language (SGML). The language that would eventually become SGML was invented by Charles F. Goldfarb, Ed Mosher, and Ray Lorie at IBM in the 1970s and developed by many people around the world until its eventual adoption as ISO standard 8879 in 1986. SGML was intended to solve many of the same problems XML solves in much the same way as XML solves them. It was and is a semantic and structural markup language for text documents. SGML is extremely powerful and achieved some success in the U.S. military and government, in the aerospace sector, and in other domains that needed ways of efficiently managing technical documents that were tens of thousands of pages long.

SGML’s biggest success was HTML, which was and is an SGML application. However, HTML is just one SGML application. It does not have anything close to the full power of SGML itself. SGML has also been used to define many other document formats, including DocBook and TEI, both of which we’ll discuss shortly.

However, SGML is complicated—very, very complicated. The official SGML specification is over 150 very technical pages. It covers many special cases and unlikely scenarios. It is so complex that almost no software has ever implemented it fully. Programs that implement or rely on different subsets of SGML are often incompatible. The special feature that one program considers essential is all too often considered extraneous fluff and omitted by the ...