CSS Minifier
Strips comments and collapsible whitespace. Conservative by design — it will not restructure your selectors.
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Worked example
A 40 KB stylesheet with comments and indentation typically compresses to 28 KB minified and 7 KB over the wire with gzip. If your server compresses, minification saves far less than the numbers suggest.
Minification removes characters the parser does not need: comments, indentation, the last semicolon in a block. It does not change what the CSS does — if the output behaves differently, the minifier is broken.
What this deliberately does not do
No selector merging, no property reordering, no colour shortening beyond what whitespace removal achieves. Those optimisations are where minifiers introduce bugs, and with gzip on, additional minification saves single-digit percentages.