CSSO [1] is a tool that removes duplicate declarations during minification instead of just warning about them. It also performs more advanced structural optimizations.
I think the purpose of this package is more about making existing source code easier to maintain (applying DRY principals to CSS) than about browser-oriented optimization (thus purpose of CSSO.)
You could keep the csscss'ed source in version control (easy to maintain), and use csso as an automated step before deploying.
css-ratiocinator [1] is also another tool someone mentioned to me after I launched cssscss.