I use a Greasemonkey script on certain sites that strips all styles (both stylesheets and inline) as well as presentational attributes and FONT tags. The script adds a 'mystyle' class to the HTML element. This stylesheet complements that script by adding my styles to the unstyled content.
I like your idea very much :)
Could you provide your GM script here (and add link to it in some comment in the userstyle) ?