In my opinion, it does already, without needing much CSS except to limit pictures to the width of the viewable area (you can click on them and select "view picture" to zoom them, if desired). I rarely need to add my own CSS code to web pages that do not already have CSS; it is those that do, that I need to fix.

I made up a attribute feature="usercss" which is supposed to mean to use the user-specified CSS (or suitable defaults for semantic HTML) instead of this one, if the client supports that feature; it can also use the user-specified CSS (or suitable defaults) if the web page does not specify any CSS at all. As far as I know no browsers implement this, but some of the people making up some newer browsers (the less common ones) might consider to add such a feature when user-specified CSS is implemented.