What about XML for content and XSLT for display? User could use a variety of XSLTs depending on constraints, viewing conditions.
Also, what's wrong with HTML/CSS that is in any way resolved by XML/XSLT? I'm genuinely curious why somebody would put their content into a format with no default, well-understood display semantics.
and the client could use the default xslt if they were happy with that, and a custom otherwise.