Minify CSS with only the safe shortenings

Three, because three are provably reversible by the browser's own parser. A six-digit hex colour whose digits pair up becomes three — #ffffff is #fff, and the eight-digit form with alpha collapses to four the same way — which is a syntax the spec defines as identical, not an approximation. A zero with one of fifteen length units on it becomes a bare 0, since a length of zero is the same length whatever unit it was written in. A leading zero before a decimal point goes, because .5rem and 0.5rem are the same token to the tokeniser. Everything else that would make the file smaller changes something about the sheet, so it is left out.

Because a unitless zero is a length, not a time, and transition and animation shorthands are parsed positionally. Write transition: box-shadow 0s ease and drop the s and the browser reads the 0 as the wrong component of the shorthand — a rule that quietly stops animating. Angles behave the same way: rotate(0deg) needs its unit, since rotate(0) is only valid because the function coerces it, and inside a transform list the coercion is not guaranteed. Percentages are excluded too, as 0% and 0 are genuinely different in a gradient stop. Only the fifteen length units lose theirs.

A custom property is a token stream waiting for substitution, not a value with a known type. --gutter: 0px 16px could be consumed by a property that needs the unit, split apart by JavaScript reading getPropertyValue, or dropped into a calc() where a bare 0 is a type error. The minifier cannot know which, and being wrong means a layout that silently collapses, so everything to the right of a -- property's colon is copied out exactly as written — spacing included. The same caution applies inside calc(), min(), max() and clamp() wherever they appear.

No, and that is the single biggest difference between this and an optimising minifier. Combining two rules that share a selector, folding four longhands into a shorthand, or dropping what looks like a duplicate declaration all assume the sheet has no order-dependent tricks in it — and real sheets are full of them: a fallback declaration written before the modern one so old browsers take the first and new ones take the second, a duplicate property used as a targeted hack, a deliberate override sitting later in the cascade. Your rules come out in your order, one for one, with the same selectors.

A quarter or so of the raw bytes, and much less than that once compression has had a look. A 517-byte hand-written theme with comments came out 28% smaller, or 17% after gzip. A 44.8 KB sheet lost 25% raw but only 5% gzipped, because it was repetitive enough that the compressor had already dealt with most of what got deleted. The spread between those two figures is the point — measure your own file rather than trusting a rule of thumb, and read the gzip cell rather than the raw one whenever the file will be served compressed.

All three come through intact. A quoted font name keeps the comma inside it, so "Foo, Bar", sans-serif does not lose the name it was pointing at. A data URI sitting inside an unquoted url() travels as a single token, payload punctuation and all, since one stray space in an unquoted URL is enough to make it invalid. content strings keep their exact characters, braces and all. !important keeps its space before the bang, and the property it is attached to keeps its place in the cascade.

An unterminated string, an unterminated /* comment, an unterminated url(), a } with no opener, or a file that simply runs out mid-block. Any of those and you get your text back unchanged with the reason named, rather than a smaller file that has lost a rule. This matters more for minifying than for formatting: a stylesheet that came back mangled will still load, apply most of itself, and fail in one place you will not find until someone reports it.