I see what you're trying to do, and I think the only way you could solve this would be with the help of
JavaScript font detection.
Ok, so it's not the *ideal* solution, but it will work in most cases (and should degrade nicely if the user has JavaScript disabled).
I've only just stumbled on this script myself, so not sure how it works exactly. However, I think it returns an array of available fonts, so you'd simply need to check if the font you're using is in that array - if it's not, serve up the new
CSS font-size, otherwise leave it be
