Coombs, Renear, and DeRose: Markup systems and the future of scholarly text processing. CACM, November 1987
Examples are MS Word and other word processing systems.
Embedding of codes in the text expressing font, size, color etc.
Is inflexible and offers poor longevity and reusability, but can be used to produce nice-looking pages.
Always used with an authoring system that hides the markup from the user. WYSIWYG is a false claim!
Examples are unix tools like Troff, TeX and PostScript.
Primitives remain presentational, but they are embedded in a procedural framework, allowing macros and subroutines, and the notion of the current graphics state can be made concrete.
Procedural markup can be authored directly by humans; much Physics and Math research is published in hand-constructed TeX or LaTeX.
Examples: XML (and its predecessor SGML).
The idea is that the markup doesn't tell you what to do with a piece of text, it tells you what it is, describes it. Another term could be "labeling."
All XML does is provide a nice flexible internationalized way to label the elements of a data structure and ship them around with the labels attached.
Descriptive markup was born in the world of publishing technology, and has many advantages for serious large-scale publishing