I think that there is merit in designing a minifier that is explicitly designed to optimise the gziped size rather than the uncompressed size.

Things like:

* Rearrange rules within the file to put similar rules within the sliding window.

* Rearrange rules so that tail of the last declaration of one rule and the start of the next selector create the longest possible common substring.

* Rearrange the order of declarations within the rules to maximize the length of common substrings that span two declarations, ie ": 2em;\nbackground-color: rgb("