accessibility and post-processing style sheets

I've been thinking that a good way to improve accessibility might be
to allow the reader to provide an extra style sheet which would get processed
after the normal cascading and inheritance happens.  That way, a reader
could write rules that match physical appearance.

Here's a rule that might be supported:

[font-size<10pt] {font-size: 10pt}

That would have the effect of forcing the minimum font size to 10pt.

But I'm not sure if this is the best approach to take; specifying a mininum
contrast between a foreground and a background within this framework
would be painful, especially if you wanted to specify that you had
a particular variety of colorblindness.