Most sites have segregated data from presentation. The problem would be your second point - that it's not exposed as being user controllable. It is in fact us

Most sites have segregated data from presentation. The problem would be your second point - that it's not exposed as being user controllable. It is in fact user controllable through scripting in most cases, but it's not surfaced in any way. Right in my browser, firefox, and maybe yours, I can set the default fonts, sizes, and colors, and I can force sites to use my settings. This is a partial step towards where you're going. Most designers will not simply use the foreground, background, serif, and sans-serif fonts that a user has set. This could be a good thing, since the majority of people don't even know that these options exists, but it is forcing a single design upon the users.