CSSO (CSS Optimizer) is a CSS minifier. It performs three sort of transformations: cleaning (removing redundant), compression (replacement for shorter form) and restructuring (merge of declarations, rulesets and so on). As a result your CSS becomes much smaller. Warning: CSSO uses early versions of CSSTree that still in active development. CSSO doesn't guarantee API behind syntax field or AST format will not change in future releases of CSSO, since it's subject to change in CSSTree. Be carefull with CSSO updates if you use syntax API until this warning removal.