This one simple protocol is understood and used by almost every PC out there. Fonts, spacing, and the formatting stuff is a binary function. Standard Generalized Markup Language, or SGML for short. SGML is, as the name indicates, a markup language. Passages of text are surrounded by delimiters known as tags.
Tags define the look of the text they contain.
The DTD simply lists what the tags are and what each one will do to the text. The most famous example of an SGML DTD is HTML, the very format this document appears in. HTML makes up a huge majority of the content of the Web. The answer lies in the way markup languages are supposed to work. You want to say something and the document is the expression of that statement.