CSSO 介绍
CSSO (CSS Optimizer) 是一个 CSS 最小化压缩工具，不仅对 CSS 做压缩还对 CSS 进行结构分析，生成更小的文件。
安全转换：
- Removal of whitespace
- Removal of trailing ;
- Removal of comments
- Removal of invalid @charset и @import declarations
- Minification of color properties
- Minification of 0
- Minification of multi-line strings
- Minification of the font-weight property
结构优化：
- Merging blocks with identical selectors
- Merging blocks with identical properties
- Removal of overridden properties
- Removal of overridden shorthand properties
- Removal of repeating selectors
- Partial merging of blocks
- Partial splitting of blocks
- Removal of empty ruleset and at-rule
- Minification of margin and padding properties