[dgallagher]

I guess you could say that HTML meets that standard. For example, <h1> through <h6> headers could be rendered by the browser accordingly; most default renderers still display things mostly the same way Netscape Navigator 2.0 did back in the Win 3.1 days (disable CSS on a site and you'll see).

Marked on the Mac, which is a Markdown renderer, loads .mmd files and renders them against a CSS template which you can swap out. That fits the MVC idiom nicely, although far from a perfect implementation as it's merely meant for viewing markdown files, not everything on the internet.