Stop zero-value minification from producing invalid CSS

[marevol]

Problem

Two zero-value minification rules in CssCompressor rewrite a zero into a form the CSS grammar does not accept in that position. The browser then drops the whole declaration, so the rule costs the declaration in order to save a byte — silently, with no warning from the build.

1. Zero-percentage stripping inside a group

input	output	why it breaks
hsla(0,0%,100%,.5)	hsla(0,0,100%,.5)	legacy comma hsl()/hsla() take <percentage> for saturation and lightness
rgb(0%,50%,100%)	rgb(0,50%,100%)	legacy comma rgb() is all-<percentage> or all-<number>, never mixed
color-mix(in srgb,red 0%,blue)	color-mix(in srgb,red 0,blue)	color-mix() takes <percentage [0,100]>
min(0%,10px)	min(0,10px)	math functions type-check their arguments: <number> vs <length>

The underlying rule — "a zero <length> may drop its unit" — is correct, but it was applied to <percentage> as well, and it fired on anything inside a (, so it reached colour and math functions it was never meant for.

2. Collapsing a run of zeroes

input	output	why it breaks
box-shadow:0 0	box-shadow:0	a <shadow> needs both offsets
text-shadow:0 0 0	text-shadow:0	same
perspective-origin:0 0	perspective-origin:0	one value means 0 center, not 0 0

margin:0 0 0 0 → margin:0 is the box-model shorthand and collapses exactly; box-shadow is not a shorthand at all. Losing a box-shadow declaration also takes down every other shadow in the same comma-separated list. flex was already excluded here for the same class of reason.

Both defects predate the fork — yuicompressor-2.4.8 from Maven Central produces the same output — and the first was already recorded in KnownCssLimitationsTest with instructions to delete the entries once fixed.

Fix

• The % branch of the in-group zero-unit stripping is split out and now consults the enclosing function name against PERCENTAGE_REQUIRED_FUNCTIONS. The name is captured rather than looked behind, so min does not also match the tail of minmax(), where a zero really may lose its unit.
• The zero-run collapse now matches on the property name (vendor prefix stripped) and skips ZERO_RUN_NOT_COLLAPSIBLE.

Unchanged: length units inside groups (translate(0px,10px) → translate(0,10px)), gradient colour stops (red 0% → red 0), minmax(0%,1fr) → minmax(0,1fr), margin/padding/border-radius/gap collapsing, and the background-position/transform-origin re-expansion.

Verification

• Corpus: 143 real-world stylesheets (Bootstrap, Font Awesome, Swiper, Vuetify, VitePress and Fess/CodeLibs themes) compressed, then every declaration checked against the CSS grammar with css-tree's lexer. Declarations that are valid in the source but invalid after compression: 26 files before, 0 after. The 16 files still flagged are var() fallbacks css-tree cannot resolve — flagged identically in the source.
• All 143 remain byte-identical when compressed a second time.
• CssColorFunctionTest (new, 20 cases) pins every corrupted value plus the neighbouring cases that must keep compressing.
• KnownCssLimitationsTest: the three percentage rows and the min() case are removed, as that file instructs. The <time>-inside-a-group case is untouched and still recorded.
• Full suite: 780 tests, 0 failures, 3 skipped.