Compress stylesheets with a CSS-aware optimiser that merges rules and shortens values, not just strips spaces.
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This runs CSSO, which does structural work rather than whitespace removal: merging duplicate rules, shortening values, and dropping declarations later ones override. That is why the saving is substantially larger than stripping spaces achieves, and why some of it survives gzip.
The transformations are safe for ordinary stylesheets. Where care is needed is CSS that depends on source order in ways a tool cannot see — hacks targeting specific browsers, or rules a later stylesheet expects to override at a particular position.