Most systems use a human-readable form, which uses special symbols to denote the markup. Guide is a relatively old hypertext system. It borrowed its markup language from an old document-processing system: troff. The hypertext commands are placed on separate lines and start with a dot.
The documentation for the popular Emacs editor is written in the Texinfo languages, which borrowed its markup language from the public document-processing package TeX.
A commonly accepted standard for markup languages is SGML, the Standard Generalized Markup Language, an ISO standard. There are two hypertext markup languages that use the SGML syntax: Hytime and HTML. Recently, a new markup language has been introduced, as a simplified version of SGML: XML (for eXtendable Markup Language).