CSSO
CSS minifier with structural optimizations
...CSSO is based on CSSTree to parse CSS into AST, AST traversal and to generate AST back to CSS. All CSSTree API is available behind syntax field extended with compress() method. You may minify CSS step by step. CSSO can use data about how CSS is used in the markup for better compression. File with this data (JSON) can be set using the usage option.