Is it possible to set the minimum value of a margin set by a percentage using CSS?

What I want to accomplish is very similar to what the min-width attribute does for width values.

I want to be able to set a margin value by a percentage up to a point, and then switch over to a concrete predefined pixel value. So the margin would be margin-left: 20%; and then I would have a min-value: 100px.

Is this possible, and if not with CSS, what sorts of workarounds are there?