HTML
By 1991, it appeared to many that SGML would be limited to commercial and data-based applications while WYSIWYG tools (which stored documents in proprietary binary formats) would suffice for other document processing applications. The situation changed when Sir Tim Berners-Lee, learning of SGML from co-worker Anders Berglund and others at CERN, used SGML syntax to create HTML. HTML resembles other SGML-based tag languages, although it began as simpler than most and a formal DTD was not developed until later. Steven DeRose argues that HTML’s use of descriptive markup (and SGML in particular) was a major factor in the success of the Web, because of the flexibility and extensibility that it enabled (other factors include the notion of URLs and the free distribution of browsers). HTML is quite likely the most used markup language in the world today.
Some would restrict the term “markup language” to systems that directly support non-hierarchical structures (see Hierarchical model). By this definition HTML, XML, and even SGML (apart from its rarely-used CONCUR option) would be disqualified and called “container languages” instead. However, the term “container language” is not in widespread use, and such hierarchical languages are almost universally considered markup languages. There is active research on non-hierarchical markup models, some expressed within XML and related languages (for example, using the Text Encoding Initiative Guidelines and derivatives such as the Open Scripture Information Standard and CLIX; and some not (for example, MECS and the Layed Markup and Annotation Language or LMNL). Much of this research is published in the proceedings of the Extreme Markup and Balisage conferences, generally held in Montreal.