In addition to the contributions of Goldfarb, Mosher and Laurie referenced above in Hugh's link to some ancient history, it would be good to consider the impact of such early programs as WordStar and WordPerfect. They brought the concept of interweaving marks into text documents to control print output into the mini- and personal computer world. While the larger industries were entrenched in their use of IBM's Document Composition Facility/SCRIPT systems, and it took awhile for businesses to dislodge dedicated word processing systems like WANG, DEC, and others form their midst, these early personal computer word processing applications became an increasingly familiar model for many in the 1980s. Though later displaced by GUI-based programs, the concepts of dot commands (WordStar) and control-codes in WordPerfect (along with its all important Reveal Codes screen that could show you exactly which markup was in play where) made the conceptual leap to other kinds of markup systems easier.
Tangentially, I wonder who would have found the move to HTML easier: those raised on page-layout systems (Page Maker, etc.) or those who used document-based systems like WordPerfect?
And for an interesting read:
"Remembering the Office of the Future: Word Processing and Office Automation before the Personal Computer" - A comprehensive history of early word processing concepts, hardware, software, and use. By Thomas Haigh, IEEE Annals of the History of Computing 28:4 (October-December 2006):6-31. http://www.tomandmaria.com/tom/Writing/Annals2006WP.pdf