The principle of any sgml'ed document (linuxdoc, docbook, html) is more or less the same:
Don't write how it should look like, but write what it is.
This is a different approach than the standard "wysiwyg"
What you see is what you (should) get (if you are a very lucky one and your computer wins the war against buggy software)one. You do not tell the programm that this line should be in a bigger font, for looking like a headline. The thing you do is telling that this line is a headline. You do not try to make your document look like a report, but you tag it to be a report.
The big advantages of this approache are:
If you ever tried the reuse a document written in a specialized wysiwy layout for a html document you know what I'm talking about.