[frandroid]

This looks awesome. If it did a second run to look up where the CSS comes from in my SASS and remove the SASS instead, that would be magical. :D

[emitstop]

Until someone makes that, you should be able to just diff the two files, see what sections were removed and go based on that.

[inostia]

Couldn't you just add a Grunt task to purify the compiled SASS?

[jonahx]

This would miss the point. Sure, it would still lighten the css file downloaded by the client, but it would do nothing to clean up the source file developers work with and thus lighten their cognitive burden, which imo is more important.