Currently there are few options for organizing and displaying a "document" in a consistent, flexible, visually appealing way.  There's always HTML/CSS which is renound for inconsistancy and unexpected results, docbook addresses the need for a documentation meta-language, but focuses on producing many different incompatible formats of output from a single source file.  There are a number of new systems that take xml and use it to produce postscript or pdf output, some of which are looking pretty cool, but fall short of being useful to more than a very narrow niche market.