[eluser]m4rw3r[/eluser]
If you want something more rigid and standardized, look at DocBook.
It is XML based and contains a huge set of tags for different purposes. The large number of tags may seem daunting at first, but it gets easier once you've learned the basic ones (one downside is that you will probably have to search for the correct tag sometimes).
Styling it should be quite straightforward, but it will be harder to change the HTML structure as it uses a XSLT stylesheet to convert the xml to html (or PDF, man pages, whatever).
If you want to take a look at a project I've written which uses docbook, look at RapidDataMapper. The xml sources are in doc/manual/src.