CSS Minifier

Removing whitespace and comments is the obvious, easy part of minification — every minifier does that, and on its own it is barely more than a find-and-replace pass. The part that separates a real CSS-aware minifier from naive whitespace stripping is everything that requires actually understanding the stylesheet: shorthand value compression, where a color written as `#ffffff` becomes `#fff` and redundant zero units get trimmed, and duplicate or overlapping rule merging, where two rules that end up applying the same declarations get combined into one. Both require parsing the CSS into a real structure first — knowing which selector applies to which declarations, and which values are equivalent — which is why csso builds an actual syntax tree rather than pattern-matching text, the same reason a real parser earns its keep over a regex pass.

Modern build tools minify their CSS output automatically as part of a production build, so for anyone working inside a bundled project, minification already happens invisibly every time they build. A standalone tool like this one covers what falls outside that pipeline: a CMS or platform with no build step at all, a quick one-off snippet that needs to fit somewhere size-constrained, or just checking how much a given stylesheet would actually shrink before deciding whether wiring up tooling is worth it. For a single file needed once, pasting it here and copying the result back out is the faster path to the same output a bundler would produce.

A CSS-aware minifier is deliberately conservative about what it will merge or rewrite: custom properties (CSS variables), media queries, keyframe animations and vendor-prefixed declarations are all preserved exactly as written, because collapsing them incorrectly could silently change how a page renders on some browsers or screen sizes rather than just how the source looks. The optimisations that are safe to make automatically — whitespace, comments, shorthand values, genuinely duplicate rules — are the ones with no possible effect on the rendered page, which is the whole design principle behind using a real parser instead of a blunter, riskier find-and-replace approach.