I'm not talking about used/unused here. I have two stylesheets and some rules differ (either in selectors, rules or both) and some are exactly the same.
So I'm looking for a way to extract and move those rules, which are the same in both files, into a third stylesheet.
In other words: an intersection of two stylesheets. Strangely, I couldn't find a software for this. Would have expected this to be a more common problem.
Background, for those who care: I'm converting a desktop website into a mobile one and I've started by duplicating the desktop stylesheet and changing it (throwing stuff out, adding to it).