[robobun]

Problem

• bun build prints invalid CSS when the unresolved alpha of a relative rgb() names a channel: rgb(from #336699 r g b / calc(alpha * var(--o))) becomes rgb(51 102 153/calc(alpha*var(--o))).
• A keyword is on the wrong scale: rgb(from #336699 calc(r + 10) g b) prints #f69.

Fix

• A keyword is a number in the range of its function, also in nested math (css: resolve relative color channel keywords as numbers in the function's range #38553, css: resolve relative color keywords inside nested math functions #39514).
• A relative color stays as written when its fold is not exact: a keyword in the alpha or in round(), mod(), rem(), sign(), hypot(), a channel or origin outside sRGB, the hue of a converted gray. With no var() it takes no color fallback, and the earlier declaration stays.
• color: var(--c, lab(60% 10 10)); color: red now shows red. Unparsed tokens no longer join: rgb(255,0,0)0% printed red0%.
• Verified: test/js/bun/css, test/bundler/css. Self-reviewed: 43 concerns raised, 13 addressed, 9 in part (Notes).

Background

• A relative color, rgb(from <origin> r g b / alpha), takes each keyword as a number from the origin.
• "As written": the declaration stays as tokens for the browser.
• Weighed: clip an out-of-range channel, as main does. Per Bun.color: return null for unconvertible colors, fix hsl/lab output, lab() and named-color parsing #33047, pass-through comes first.
• css: parse number and percentage channels in lab(), lch(), oklab(), oklch(), hsl() and hwb() #45084 shares three commits.

Downsides

• A color that stays as written shows nothing in a browser with no relative colors. Main printed a static color.
• Per unparsed value: about +43 instructions per token, +79 to +391 for fallbacks. Per typed declaration: +15 to +37. Release .text: +15,872 B.
• A variable that holds a keyword still breaks rgb(from #336699 r g b / var(--o)).

Notes

Rule. A relative color is folded only when a browser computes the same color from the source. In every other case the declaration is printed as written. #39514 alone turns valid output into wrong colors, so #38553 and #39514 are one change here.

Commits

1. keep the tokens of an unparsed value apart. TokenList::to_css checks each pair of neighbors against the serialization table of CSS Syntax 3 and writes one space where they would join. It replaces the / * special case of css: keep '/' and '*' delims separated when minifying token lists #33683. rgb(255,0,0)0% printed red0%, solid#f00 printed solidred, a/**/b printed ab, 2n+1 printed 2n1. An integer keeps its plus sign: 2n +1 printed 2n 1.
2. keep the origin alpha. The color half of css: keep rotate: 0deg out of none, and keep the origin alpha in relative colors #38486. rgb(from #0008 r g b) printed #000.
3. as written when the unresolved alpha names a channel. The first problem above.
4. do not resolve ... whose origin is outside the sRGB gamut. The first commit of css: parse number and percentage channels in lab(), lch(), oklab(), oklch(), hsl() and hwb() #45084, same source lines.
5. give a relative color with no var() no color fallback as tokens. Commit 4 leaves a color for the browser that was a static color. As tokens it would take the fallback of an unparsed value: another origin in the style rule, and a copy in an @supports rule, which wins over a later declaration. A relative color with var() keeps the fallback that it has on main. Five more parts keep the cascade of the source. The earlier color or text-shadow stays before such a color. An earlier rule with the same selector stays. A color after it still takes its own fallbacks (color: rgb(from color(display-p3 0 1 0) r g b); color: lab(50% 10 10) prints #8c7166 between the two). light-dark() around it is printed as written. A declaration, unparsed or typed, follows an earlier declaration of its property into that declaration's @supports rules.
6. as written when a channel leaves its range. rgb(from #336699 300 g b) printed #f69. A browser computes color(srgb 1.17647 0.4 0.6). Bun.color() has no form for such a color. It parses with ParserOpts::MUST_FOLD, which turns each of these rules off, so its values are the ones of main. One value differs: an rgb() that is painted as it is reads an origin outside sRGB with no gamut mapping and clips (#0f0 for rgb(from color(display-p3 0 1 0) r g b)). The origin of another color and an operand of color-mix() are not painted (ParserOpts::INNER_COLOR). They are read as on main.
7. keep a relative color that is left for the browser in a typed @property. A typed initial-value has no unparsed form, so such a color was a build error. A value that fails is read again with MUST_FOLD. When that parse succeeds, the value is kept as tokens. Other values are errors as before.
8. clamp the lightness ... at 100%. lab(150% 20 30) is lab(100% 20 30) in a browser.
9. as written when it takes the hue of a converted gray. oklch(from #fff 82.5% 0.1 h) printed hue 90. Chrome 153 computes 23.8, and CSS Color 4 makes the hue missing. hsl(from lab(50% 0 0) h 100% 50%) printed #0ff.
10. resolve relative color channel keywords as numbers in the function's range. The change of css: resolve relative color channel keywords as numbers in the function's range #38553. A calculation that is NaN is 0 in a channel, as in Chrome. It was none.
11. fail a percentage calc that cannot be reduced. One source line of css: parse calc(<channel> * <percentage>) in relative colors #38513. opacity: calc(50% + 1) printed opacity:0.
12. resolve relative color keywords inside nested math functions. The change of css: resolve relative color keywords inside nested math functions #39514. A keyword does not reach round(), mod(), rem(), sign() or hypot(), at the top of a channel or deeper.

After the review on this PR

• Bun.color() returned null for colors that main resolved. It now gives the value of main for them (MUST_FOLD).
• color: red; color: rgb(from #336699 300 g b) lost red. The earlier declaration now stays, and so does an earlier rule of the same selector.
• A relative color followed into every @supports rule of its style rule, and there it won over later declarations. A declaration now follows only an earlier declaration of its own property, with the fallback that the rule can read.
• The typed @property check used a counter that one unrelated relative color could satisfy. The counter is gone.
• The rem() change of css: fold rem() with a truncated quotient, so it keeps the sign of the dividend #32286 is no longer carried: a % b folds rem(25%, 5%) to 5% and prints rem(-0.3px, 0.1px) with two minus signs. Its findings went to that PR.
• none as the alpha of an origin: no change. Chrome 153 computes color(srgb 0 0 0 / 0) for rgb(from rgb(0 0 0 / none) r g b), as the fold does.
• Second round: a color after a relative color that stays as written took no fallback of its own. The handlers of color, text-shadow, background, border, border-image and box-shadow no longer count such a declaration as the fallback. An arm of light-dark() is painted as it is in Bun.color().
• Third round: a typed declaration follows an earlier declaration into its @supports rules too. color: var(--c, lab(60% 10 10)); color: red showed the first color in a browser with lab(), on main too. A relative color that now folds is such a typed declaration. The follow has one place for typed declarations (DeclarationHandler::handle_property) and one for unparsed ones (add_unparsed_fallbacks). Rules that merge are minified again as one block, and nothing follows then. The @supports rules that are staged at that time belong to a later rule.
• Fourth round: an integer that is written with a plus sign keeps it in a token list. 2n +1 printed 2n 1, on main too, and :nth-child() does not read that. --x: +1 now prints +1.

Open PRs that this PR needs or touches

• css: keep an unparsed width/height/color after the declarations written before it #42101 appends every unparsed color or text-shadow after the earlier declaration. Commit 5 does that for a relative color with no var() only, on the same line of prefix_handler.rs.
• css: print negative numbers below 1 with a single minus sign #38516: a number in (-1e-6, 0) or in (-1, -0.9999995] prints with two minus signs, on main too: lab(from #336699 l calc(a * 0.000000001) b) prints lab(41.5208% --4.57311e-9 -33.4942) on both.
• css: fold rem() with a truncated quotient, so it keeps the sign of the dividend #32286 (rem()), css: fold one-argument hypot() to the absolute value of its argument #38639 (hypot()), css: fold round() by choosing between the lower and upper multiple of |B| #45249 (round()): not carried. A keyword does not reach these functions, so they fold numbers only, as on main.
• css: fold min(), max() and clamp() over plain numbers #39515 is not carried. min(), max() and clamp() over keywords behave as on main: as written, or folded by the older percentage reading (min(r, alpha) is wrong there, as on main).

Options for a maintainer

• Commit 3 keeps the fold of rgb(from X r g b / var(--a)) to rgb(R G B / var(--a)). That fold is wrong when the variable holds a channel keyword. The alternative is to never fold a relative rgb()/hsl() whose alpha is unresolved. The other functions are already printed as written in that case.
• Commit 10 keeps two older readings as a last attempt: a keyword as a percentage (calc(r + 10%) folds to #4d6699) and a keyword as an angle (calc(h + 30deg)). A browser rejects both inputs. The docs showed calc(l + 15%) until this PR. lightningcss removed the percentage reading in 1.30.0. Deleting both is about 30 lines and breaks that docs example for its users.
• Commit 12 folds no step function of a keyword. Main folds one whose keyword is written directly as its argument. In the sweep below, 2,415 declarations hold one of the five functions: main folds 561 of them, 189 wrong (rgb(from #336699 round(r, 10) g b) prints #069). The PR leaves 348 of the right ones and 167 of the wrong ones as written. With the keyword on its scale the fold is right for an exact keyword (r of a hex origin) and wrong near a step for a converted one: hsl(from hsl(210 50% 50%) rem(h, 30) s l) has the hue 29.7 where Chrome computes 0. The alternative is to fold when the keyword is exact.
• A relative color with var() keeps the fallback of main: the origin as an sRGB color in the style rule, and an @supports rule with the color as written. No browser reads the first form. Dropping it is right only when each later declaration that overrides the property follows into the @supports rules. A shorthand or a longhand of the property does not follow yet (see "Seen on main").
• css: parse number and percentage channels in lab(), lch(), oklab(), oklch(), hsl() and hwb() #45084 and this PR: one PR or two, and which first. The shared commits are 2, 4 and 11, so the second to merge needs a rebase. Commit 10 and the reader of css: parse number and percentage channels in lab(), lch(), oklab(), oklch(), hsl() and hwb() #45084 change the same functions. Commit 1 has no dependency on relative colors and can be its own PR.
• light-dark() around a relative color with no var() is printed as written, with no var(--buncss-light, ...) fallback form. A browser with relative colors and no light-dark() (Chrome 119 to 122, Safari 16.4 to 17.4) then drops the declaration and takes the one before it, where the fallback form of main worked. With the fallback form, a browser with no relative colors keeps a declaration that it cannot compute, and the declaration before it is lost. The targets cannot decide it: they hold both kinds of browser.
• Bun.color() for a relative hsl() or hwb() that a style sheet keeps as written: the clamped and gamut-mapped color of main. An exact value needs an unclamped hsl() conversion, which does not exist yet. An rgb() inside another function is gamut mapped as on main too, where a browser hands it on unclamped.

Against a browser. Release builds of main (91e7e3b) and of this PR. Each output is loaded in headless Chrome 153 next to its source, and the computed colors are compared with a tolerance of 1/255.

11,750 generated declarations: ten functions, 146 literal origins, keyword, number, percentage and none channels, calc() and 20 math functions at the top of a channel and nested, 38 alpha forms, and the wrappers color-mix(), light-dark(), a nested origin, var(--x, V) and a gradient. Chrome accepts 11,482 of the sources.

	invalid	wrong	as written	right
main	53	1,515	5,832	4,082
PR	1	238	6,048	5,195

• Right on main and wrong or invalid in the PR: 0.
• Right on main and as written in the PR: 377. These are the step functions (348) and the cases of the rule.
• As written on main and wrong in the PR: 37. Each is a fold that main has for the values it already folds, reached now because a number can set the lightness and a keyword resolves in nested math: oklab()/oklch() at lightness 0 or 1 with chroma, whose lab() fallback is outside 0-100% (12, handed off), the hue of a gray lch() result with the same paint (8), color-mix() around an operand that now folds (13), an origin rounded to 8 bits (3), an infinite hue (1).
• The one invalid value is a number with two minus signs inside a var() fallback (css: print negative numbers below 1 with a single minus sign #38516), the same output as on main.
• 1,105 more rules put the value next to other declarations of color. Main: 16 invalid, 548 wrong, 145 as written, 396 right. PR: 2, 32, 599, 472. As written on main and wrong in the PR: 1 (color-mix()).

Cascade: 10,802 generated rules with 2 to 6 declarations (typed values, var() with a wide-gamut fallback, relative colors, shorthand and longhand pairs, !important), three target sets, five browser classes. With the targets of bun build and a modern browser, main shows another declaration than the source for 2,804 of the rules, and the PR for 705. Each of the 705 holds a shorthand and a longhand of it, and main is wrong in each of them too. A rule counts below when, for one property, main shows the declaration that the source makes win and the PR shows another.

• 6,802 systematic rules: 0.
• 4,000 random rules, modern browser: 3. Each is the order bug of "Seen on main", which main escapes there only because the variable is not set.
• A second corpus has 4,200 rules. 3,200 of them hold a typed declaration after a var() fallback of the same property. In a modern browser, a later declaration of the same property loses to an @supports rule in 2,110 rules on main and in 0 in the PR. Main is right and the PR is not in 23 rules there. 13 are the shorthand limit below. In the other 10, main is right only while a variable is not set.
• 12,237 style sheets of 2 to 10 rules: rules that merge, nested rules, at-rules, selector lists that are split. No @supports rule holds a declaration of another rule. Main is right and the PR is not in 9 sheets. 8 need compiled logical borders, which bun build does not have. 1 is the sRGB fallback that a color after a kept relative color now takes.
• 4,000 random rules, a browser with no relative colors: 10 or 11 per class. 9 or 10 hold var(--x, <relative color>), where main printed a static fallback (the first downside). The others are a relative color with !important that stays as written, the sRGB fallback of a later color(display-p3 ...) where main printed none, and 1 of the 3 above.

Bun.color(), 387,803 strings in four output formats: 26,329 values go from wrong to the painted color, and 18,306 from null to it. 36 values were within 1/255 of Chrome on main and are not now: an hsl(), hwb() or inner rgb() of an origin outside sRGB with keyword math, where main read the keyword on the wrong scale and matched by accident. 80 valid strings go from a value to null: a division by exactly zero that the keyword scale moved (calc(1 / (r - 255))), and a keyword next to min()/max() of numbers (commit 11). No crash.

Typed @property, 45 syntaxes by 509 values: main builds and the PR fails for 92 pairs, 89 of them invalid in Chrome (a stray ), calc(10% + 5)). The 3 valid ones: calc(abs(-10%) + 5%) for <percentage> (2, main printed 0%) and a list with url() after a gradient that stays as written (1). Main fails and the PR builds for 156.

Bun.color() and the @property matrix were measured before the follow of typed declarations, which is in the declaration handlers. These two do not reach the handlers.

Tokens. 112 token spellings, every ordered pair, in a custom property. The output is read back with a port of the tokenizer. Measured before the review changes. The serializer has two changes since: a byte search, and the plus sign of an integer, which is now kept. The counts below do not include the second one.

• Written with nothing between them, 8,531 pairs are two tokens in the source. Main prints 995 of them so that they read back as other tokens. This PR prints 0 wrong, 7,518 with the bytes of main, and 18 with a space that main does not have and does not need: a number with a decimal point before a percentage below 1% (1.5 .5%), and @ before a negative number.
• Written with a comment between them (12,544 pairs): main 4,607 wrong, this PR 0.

Measured (gdb breakpoint hit counts and single-step counts, release builds of main 91e7e3b and of this PR)

• release .text: 81,665,765 -> 81,681,637 B (+15,872). .rodata is the same.
• add_unparsed_fallbacks, once per unparsed declaration of color, text-shadow, background, border, border-image and box-shadow, instructions: color:var(--c) 130 -> 209. border:1px solid var(--b) 178 -> 413. background:linear-gradient(90deg,var(--a),#fff 50%) 282 -> 537. box-shadow:0 0 0 1px var(--a),0 1px 2px rgb(0 0 0/.5) 355 -> 746. color:var(--c,color(display-p3 1 0 0)) 13,195 -> 13,318. The new work is one pass over the tokens up to the first var().
• DeclarationHandler::handle_property, once per declaration, instructions for a typed one: width:1px 281 -> 296. margin:0 615 -> 631. display:block 714 -> 743. color:red 974 -> 1,010. text-shadow:0 0 4px red 1,083 -> 1,120. The new work is one test of an empty list. In a rule that has @supports rules, each later typed declaration also looks through them. After color:var(--c,lab(60% 10 10)), width:1px is 281 -> 375. color:red, which follows into two rules, is 975 -> 1,366. The look is at one entry per staged property. A rule of 100,000 declarations after such a var() builds in 0.3 s (main 0.2 s).
• Tokenizer calls and heap allocator calls per rule with one declaration (the difference between 110 and 10 rules, divided by 100):

declaration	tokenizer	heap
color: rgb(12 34 N / var(--a))	34 -> 34	15.2 -> 15.2
color: rgb(12 34 N / .5)	13 -> 13	6.2 -> 6.2
color: rgb(from #3306NN r g b)	13 -> 13	6.2 -> 6.2
color: rgb(from #3306NN r calc(g + 10) b)	19 -> 19	7.2 -> 7.2
color: red; color: var(--cN)	14 -> 14	8.2 -> 8.2
--yN: 10px 20px 30px 40px	13 -> 13	7.2 -> 7.2
margin: var(--aN) 10px	11 -> 11	8.2 -> 8.2
color: rgb(from #336699 r g N / var(--o)), still folded	40 -> 43	15.2 -> 16.2
color: rgb(from #336699 r g N / calc(alpha * var(--o))), now as written	130 -> 159	28.2 -> 33.2
color: rgb(from #3366NN 300 g b), now as written	13 -> 37	6.3 -> 6.0

• Printing one unparsed value, instructions: --y:10px 20px 30px 40px 3,849 -> 4,150. --n:1 2 3 2,046 -> 2,260. --z:1.5 2.5 3.5 2,614 -> 2,828. margin:var(--a) 10px 1,324 -> 1,389. --x:1px solid #fff 2,232 -> 2,557. background:linear-gradient(90deg,var(--a),#fff 50%) 3,354 -> 3,787. box-shadow:0 0 0 1px var(--a),0 1px 2px rgb(0 0 0/.5) 5,940 -> 6,847.
• rgb(from red r g b / nested 16/32/64/128 deep around var(--o): 5,142 / 19,510 / 75,894 / 299,254 tokenizer calls (main: 2,214 / 8,006 / 30,342 / 118,022).
• Existing expected strings changed: the 3 display-p3 cases of relative_color_out_of_gamut.test.ts (commit 4), none elsewhere.

Known limits

• A folded rgb(), hsl() or hwb() is a legacy color. A gradient or a transition through it interpolates in sRGB. The relative color of the source interpolates in Oklab. This is so on main for every relative color that folds.
• An origin that is written as hsl(), hwb() or rgb() is rounded to 8 bits before its keywords are read, as on main: hsl(from hsl(30 2% 97%) h 60% 50%) prints #c33. A color that stays as written prints that rounded origin too.
• A gray that is computed (a nested relative color, color-mix()) or read through var() keeps the hue of the rounding error, as on main. So does the gray result of oklch(from #fff l c h) when it is mixed.
• Black, lab(50% 0.001 0) in lch() and a gray color(srgb-linear ...) in hsl() have the hue 0 in Chrome 153 and stay as written.
• 1e39 / 1e39 is NaN in f32 and folds to 0. asin(2) in a channel folds to 0, and a browser rejects it. A division by exactly zero is not folded, as on main.
• The fold does not look at the browser targets. A color that stays as written has no value for a browser without relative colors.
• A relative color that now folds is a typed declaration. A shorthand does not follow an earlier longhand into its @supports rule, or the reverse, on main too. So background-color: var(--u, lab(60% 10 10)); background: oklch(from lch(50% 50 30) 0.8 c h) shows the first color. Main left that relative color as tokens, with a rule of its own.
• After a var() declaration, the border handler gives a later typed color no sRGB fallback. It also drops a typed border between the two. Main does both. A relative color that now folds to lab() or oklch() is such a typed color: border: 11px solid var(--v, lab(61% 10 10)); border: 12px solid #b21314; border: 13px solid oklch(from lch(50% 50 60) 0.8 c h) shows the first border in a browser with no oklch(). Main shows the second.
• A typed light-dark() that follows into an @supports rule is printed as in the style rule, with the two --buncss-* variables. A bundle with no color-scheme declaration does not define them. The declaration then computes to nothing, as it does in the style rule on main. Main showed the earlier declaration in a browser with lab().
• Each later declaration of the property is in the @supports rules. That includes one that the style rule drops because the next one replaces it: color: var(--c, lab(60% 10 10)); color: red; color: blue prints red and blue there.
• Two more cases need targets that bun build does not have. With Chrome 90 alone, no target reads display-p3 without lab(). The display-p3 rule can then be printed after the lab() rule, and a later declaration shows with its sRGB fallback. With Chrome 60, logical borders are compiled. The :dir() rules lose !important on main, and a normal declaration follows an important one there.
• color: red; color: rgb(from #336699 300 g b); color: blue prints red and blue. The third declaration replaces the second, and the first is left over.
• The arguments of an unknown pseudo-class still print .5 as 0.5 when not minified (:-moz-foo(a.5)).

Seen on main and not fixed here

• .a{color:var(--c,lab(60% 10 10));color:red}: the @supports rule of the first declaration wins over red. Commit 5 fixes this order for a later declaration of the same property. It does not fix it for a shorthand or a longhand of the property (background-color: var(--c, lab(60% 10 10)); background: red).
• .d{background:var(--x,#ff2e84)}.e{background:var(--x,color(display-p3 1 0 .5))} merges the two rules and prints @supports (color:color(display-p3 0 0 0)){{background:...}}, a rule with no selector.
• .a{border-block-start-color:var(--x)} prints nothing for a target without logical borders (Chrome 60). A relative color that stays as written takes this path too. bun build has no such target.
• hsl(210 150% 40%) prints #06c. Chrome computes rgb(0, 102, 255).
• oklab(100% -0.23384 0.179502) prints the fallback lab(103.379% -79.2077 77.3233), which a browser clamps (handed off).
• .a{color:#369}.b{margin:0}.a{color:foo(1 2 3)} removes the first rule, for any unparsed value. Commit 5 keeps it for a relative color.
• .a{border-image:none} prints .a{border-image:}.
• @supports (display:grid){.a{color:red}}@supports (display:grid){.b{color:green}} prints the first rule only.
• .a{color:red!important}.b{color:green}.a{color:blue} removes the first rule.
• .a{color:blue;all:unset;color:red} prints .a{color:red;all:unset}.
• .a{color:red;color:1px inherit} prints .a{color:1px inherit}.
• Two @media rules of one query merge and are minified again. Each @supports rule of a color fallback is then nested in a copy of itself.

lightningcss 1.33.0, run on the same inputs: it has the first problem above (rgb(51 102 153/calc(alpha * var(--o)))), the origin alpha (#000), the joined tokens (red0%, ab, 2n1) and the gray hue (oklch(82.5% .1 90)). It clips rgb(from #336699 300 g b) to #f69 and gamut-maps an origin outside sRGB. It has the keyword scale and the rem() sign.

Self-review. The last full round raised 43 concerns. Addressed: the separator before a number that starts with its decimal point, the restored plus sign, the color fallback of a kept relative color, Bun.color() for rgb(), the typed @property, the gray hue of an origin of the same type, the hue in hsl() and hwb(), NaN as none, the docs sentence, stale comments. In part: the dropped color fallback, the math functions that a keyword reaches, the trackers for inherited bugs. Four smaller checks of later changes, the last two after the reviews on this PR, found more regressions: the copy of a relative color in unrelated @supports rules, the rem() change, a top-level round() or hypot() over a nested keyword, the light-dark() fallback form, an earlier rule that was removed, the clip of an inner rgb() in Bun.color(), a declaration of a merged rule that followed into the @supports rule of a later rule, and a test for a follow whose time grew with the square of the declarations of a rule. Those are fixed, or listed above. Not done, with the reason:

• One channel reader for this PR and css: parse number and percentage channels in lab(), lch(), oklab(), oklch(), hsl() and hwb() #45084, the order of the two, and commit 2 as its own PR: these need the packaging decision above.
• Delete the older readings: it breaks the old docs example, see the options.
• Fold to color(srgb ...) so that interpolation is kept: it changes every folded relative color and the typed formats of Bun.color().
• Port the WPT relative color files as a data test: not done.

Other PRs. #38553 and #39514 are closed into this PR. #38513 and #38639 sit on the closed branch of #38553 and are not carried. #31924, #31918, #32286 and the rotate half of #38486 are not part of this PR.

Suites run

• bun bd test (debug, ASAN), after the rebase on main 2d6d445: test/js/bun/css/css.test.ts, color.test.ts, token-list-backtracking.test.ts and test/bundler/css/wpt/relative_color_out_of_gamut.test.ts, 2,814 pass, 0 fail.
• A debug build on main 71bc818, all of test/js/bun/css, test/bundler/css and test/bundler/esbuild/css.test.ts: 3,200 pass. Six tests time out on this debug build. The 4 of css-fuzz.test.ts run up to 1,000 Bun.build calls in 10 s, and one call takes 56 ms there. 2 of nested-selector-expansion.test.ts take 3.2 to 4.4 s when the file runs alone, against a limit of 5 s.
• Release build, before the rebase (main 91e7e3b): the same suites, 3,197 pass, 0 fail.
• A release build of main 91e7e3b fails 258 tests in the four changed test files.

---

[human-review] gate passed · iteration 0 · 21 files touched

fails on main (without fix)

ASAN without fix: 263 failed, 73 skipped
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/pr_gate.xml" test/bundler/css/wpt/relative_color_out_of_gamut.test.ts test/js/bun/css/color.test.ts test/js/bun/css/css.test.ts test/js/bun/css/token-list-backtracking.test.ts
bun test v1.4.4 (367d939d9)

test/js/bun/css/color.test.ts:
�[38;2;255;0;0m[object Object]
(pass) console.log(color({"r":255,"g":0,"b":0}, "ansi-24bit")) [4.11ms]
�[38;5;196m[object Object]
(pass) console.log(color({"r":255,"g":0,"b":0}, "ansi-256")) [1.86ms]
�[91m[object Object]
(pass) console.log(color({"r":255,"g":0,"b":0}, "ansi-16")) [1.26ms]
(pass) color({"r":255,"g":0,"b":0}, "{rgb}") = {"r":255,"g":0,"b":0} [2.76ms]
(pass) color({"r":255,"g":0,"b":0}, "ansi-24bit") [14.00ms]
(pass) color({"r":255,"g":0,"b":0}, "ansi-16") [2.93ms]
(pass) color({"r":255,"g":0,"b":0}, "ansi256") [1.79ms]
�[38;2;0;255;0m[object Object]
(pass) console.log(color({"r":0,"g":255,"b":0}, "ansi-24bit")) [0.39ms]
�[38;5;46m[object Object]
(pass) console.log(color({"r":0,"g":255,"b":0}, "ansi-256")) [0.18ms]
�[92m[object Object]
(pass) console.log(color({"r":0,"g":255,"b":0}, "ansi-16")) [0.17ms]

... (truncated)

release without fix: 287 failed, 72 skipped
bun test v1.4.3-canary.1 (367d939d9)

test/js/bun/css/color.test.ts:
�[38;2;255;0;0m[object Object]
(pass) console.log(color({"r":255,"g":0,"b":0}, "ansi-24bit")) [7.21ms]
�[38;5;196m[object Object]
(pass) console.log(color({"r":255,"g":0,"b":0}, "ansi-256")) [0.17ms]
�[91m[object Object]
(pass) console.log(color({"r":255,"g":0,"b":0}, "ansi-16")) [0.02ms]
(pass) color({"r":255,"g":0,"b":0}, "{rgb}") = {"r":255,"g":0,"b":0} [0.39ms]
(pass) color({"r":255,"g":0,"b":0}, "ansi-24bit") [14.14ms]
(pass) color({"r":255,"g":0,"b":0}, "ansi-16") [0.12ms]
(pass) color({"r":255,"g":0,"b":0}, "ansi256") [0.02ms]
�[38;2;0;255;0m[object Object]
(pass) console.log(color({"r":0,"g":255,"b":0}, "ansi-24bit")) [0.02ms]
�[38;5;46m[object Object]
(pass) console.log(color({"r":0,"g":255,"b":0}, "ansi-256"))
�[92m[object Object]
(pass) console.log(color({"r":0,"g":255,"b":0}, "ansi-16"))
(pass) color({"r":0,"g":255,"b":0}, "{rgb}") = {"r":0,"g":255,"b":0} [0.02ms]
(pass) color({"r":0,"g":255,"b":0}, "ansi-24bit")
(pass) color({"r":0,"g":255,"b":0}, "ansi-16")
(pass) color({"r":0,"g":255,"b":0}, "ansi256")
�[38;2;0;0;255m[object Object]
(pass) console.log(color({"r":0,"g":0,"b":255}, "an
... (truncated)

passes on PR (with fix)

ASAN with fix: 73 skipped
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/pr_gate.xml" test/bundler/css/wpt/relative_color_out_of_gamut.test.ts test/js/bun/css/color.test.ts test/js/bun/css/css.test.ts test/js/bun/css/token-list-backtracking.test.ts
bun test v1.4.4 (367d939d9)

test/js/bun/css/color.test.ts:
�[38;2;255;0;0m[object Object]
(pass) console.log(color({"r":255,"g":0,"b":0}, "ansi-24bit")) [4.55ms]
�[38;5;196m[object Object]
(pass) console.log(color({"r":255,"g":0,"b":0}, "ansi-256")) [2.07ms]
�[91m[object Object]
(pass) console.log(color({"r":255,"g":0,"b":0}, "ansi-16")) [1.49ms]
(pass) color({"r":255,"g":0,"b":0}, "{rgb}") = {"r":255,"g":0,"b":0} [3.26ms]
(pass) color({"r":255,"g":0,"b":0}, "ansi-24bit") [11.41ms]
(pass) color({"r":255,"g":0,"b":0}, "ansi-16") [2.33ms]
(pass) color({"r":255,"g":0,"b":0}, "ansi256") [2.44ms]
�[38;2;0;255;0m[object Object]
(pass) console.log(color({"r":0,"g":255,"b":0}, "ansi-24bit")) [0.38ms]
�[38;5;46m[object Object]
(pass) console.log(color({"r":0,"g":255,"b":0}, "ansi-256")) [0.20ms]
�[92m[object Object]
(pass) console.log(color({"r":0,"g":255,"b":0}, "ansi-16")) [0.70ms]

... (truncated)

release with fix: 72 skipped
$ bun scripts/build.ts --profile=release
[configured] bun-profile → bun (stripped)
  target       linux-x64-gnu
  build type   Release
  build dir    ./build/release
  revision     23b06bbc2b
  features     lto, baseline

23 deps, 228 codegen, 1218 objects in 2410ms

ninja: Entering directory `/workspace/bun/build/release'
[1/4] fetch rust-argon2
[rust-argon2] up to date
[2/4] fetch lolhtml
[lolhtml] up to date
[2/4] cargo plan → /workspace/bun/build/release/rust-target/plan.json
270 units: 194 lib, 16 proc-macro (host), 21 custom-build (host), 17 run custom-build, 17 lib (host), 4 run custom-build (host), 1 rlib
[3/4] reconfigure
[1/1577] mkdir stamps
[2/1577] mkdir codegen
[3/1577] gen bake.{client,server,error}.js
-> bake.client.js, bake.server.js, bake.error.js
[4/1577] install /workspace/bun
bun install v1.4.3-canary.1 (367d939d9)

Checked 22 installs across 44 packages (no changes) [4.04s]
[5/1577] install /workspace/bun/packages/bun-error
bun install v1.4.3-canary.1 (367d939d9)

Checked 1 install across 2 packages (no changes) [97.00ms]
[6/1577] rustc unicode_xid 
[7/1577] rustc unicode_ident 
[8/1577] gen string-map defines_table
[9/1577] rustc heck 
[10
... (truncated)

diff hotspot

docs/bundler/css.mdx                               |  12 +-
 src/css/context.rs                                 |  61 +-
 src/css/css_parser.rs                              | 138 +++
 src/css/declaration.rs                             |   7 +
 src/css/properties/background.rs                   |   6 +-
 src/css/properties/border.rs                       |  25 +-
 src/css/properties/border_image.rs                 |  11 +-
 src/css/properties/box_shadow.rs                   |   4 +-
 src/css/properties/custom.rs                       | 311 +++++--
 src/css/properties/prefix_handler.rs               |  34 +-
 src/css/rules/property.rs                          |  15 +-
 src/css/rules/style.rs                             |  10 +-
 src/css/values/angle.rs                            |  11 +-
 src/css/values/calc.rs                             |  25 +-
 src/css/values/color.rs                            | 572 ++++++++----
 src/css/values/time.rs                             |  32 +-
 src/css_jsc/color_js.rs                            |   2 +-
 .../css/wpt/relative_color_out_of_gamut.test.ts    |  17 +-
 test/js/bun/css/color.test.ts                      | 217 +++++
 test/js/bun/css/css.test.ts                        | 987 +++++++++++++++++++++
 test/js/bun/css/token-list-backtracking.test.ts    |  14 +
 21 files changed, 2196 insertions(+), 315 deletions(-)

gate history · 2 passed · 0 rejected · iteration 0

evidence per changed file

file                                  reads  edits  tests
docs/bundler/css.mdx                      0      0    327
src/css/context.rs                        3      1    306
src/css/css_parser.rs                     1      1    306
src/css/declaration.rs                    1      0    304
src/css/properties/background.rs          0      0    304
src/css/properties/border.rs              0      0    304
src/css/properties/border_image.rs        0      0    304
src/css/properties/box_shadow.rs          0      0    304
src/css/properties/custom.rs              7      1    305
src/css/properties/prefix_handler.rs      1      0    305
src/css/rules/property.rs                 0      0    304
src/css/rules/style.rs                    0      0    304
src/css/values/angle.rs                   0      0    304
src/css/values/calc.rs                    2      1    306
src/css/values/color.rs                   5      0    309
src/css/values/time.rs                    0      0    304
(+ 5 more files)

[robobun]

^{Updated 8:59 AM PT - Oct 21st, 2026}

❌ @robobun, your commit 23b06bb has 1 failures in Build #130066 (All Failures):

• step failed outside runner - exit 1 on 🍎 27 aarch64 - test-bun

---

🧪   To try this PR locally:

bunx bun-pr 45362

That installs a local version of the PR into your bun-45362 executable, so you can run:

bun-45362 --bun

[robobun]

Status: ready for review. It needs a maintainer's decision on packaging (see "Options for a maintainer" in the PR body).

• Reproduced on bun 1.4.3 and on main with bun build ./in.css --outdir=out and .a { color: rgb(from #336699 r g b / calc(alpha * var(--o))); }. The output is rgb(51 102 153 / calc(alpha * var(--o))). Headless Chrome 153 computes the input to color(srgb 0.2 0.4 0.6 / 0.5) with --o: .5 and drops the output.
• This PR: css: fold relative colors on the scale of their function, and keep them as written when the fold is not exact #45362. It replaces css: resolve relative color channel keywords as numbers in the function's range #38553 and css: resolve relative color keywords inside nested math functions #39514. It carries the relative color half of css: keep rotate: 0deg out of none, and keep the origin alpha in relative colors #38486, one line of css: parse calc(<channel> * <percentage>) in relative colors #38513 and the first commit of css: parse number and percentage channels in lab(), lch(), oklab(), oklch(), hsl() and hwb() #45084. It does not carry css: fold rem() with a truncated quotient, so it keeps the sign of the dividend #32286 or css: fold min(), max() and clamp() over plain numbers #39515 any more.
• css: parse number and percentage channels in lab(), lch(), oklab(), oklch(), hsl() and hwb() #45084 overlaps this PR (three shared commits, the same channel readers). The PR body says what happens to each when the other merges first.
• css: keep an unparsed width/height/color after the declarations written before it #42101 and css: print negative numbers below 1 with a single minus sign #38516 fix two defects of main that this PR reaches. The PR body has the examples.

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• src/css/context.rs
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• src/css/properties/custom.rs
• src/css/properties/prefix_handler.rs
• src/css/values/calc.rs
• src/css/values/color.rs
• test/js/bun/css/color.test.ts

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Walkthrough

The pull request updates CSS relative-color parsing, evaluation, and serialization. It preserves expressions that cannot be resolved statically, adjusts fallback generation and declaration ordering, and adds tests and documentation for color channels, calculations, fallbacks, and token boundaries.

Changes

Relative CSS color processing

Layer / File(s)	Summary
Relative-color parsing and evaluation src/css/css_parser.rs, src/css/values/color.rs, src/css/values/calc.rs, src/css/rules/property.rs, src/css_jsc/color_js.rs	Parser options and color conversion now handle relative channel types, channel ranges, origin alpha, gamut limits, hue availability, and calculations. Math parsing and typed-property handling also update folding behavior.
Preserving unresolved colors and fallback order src/css/context.rs, src/css/declaration.rs, src/css/properties/custom.rs, src/css/properties/prefix_handler.rs, src/css/properties/background.rs, src/css/properties/border.rs, src/css/properties/border_image.rs, src/css/properties/box_shadow.rs, src/css/rules/style.rs, test/bundler/css/wpt/*	Relative colors that cannot be resolved remain as tokens in supported cases. Fallback generation and declaration ordering account for unresolved relative colors. WPT expectations now preserve out-of-gamut relative declarations.
CSS token and value serialization src/css/css_parser.rs, src/css/properties/custom.rs, src/css/values/color.rs, src/css/values/angle.rs, src/css/values/time.rs	Custom-property serialization tracks token boundaries. Color output selection and angle and time unit selection use serialization helpers.
Behavior tests and documentation test/js/bun/css/color.test.ts, test/js/bun/css/css.test.ts, test/js/bun/css/token-list-backtracking.test.ts, docs/bundler/css.mdx	Tests cover color conversion, calculations, unresolved expressions, fallback behavior, and token separation. Documentation describes conditional evaluation and retained expressions.

Suggested reviewers: jarred-sumner

Priority: ➖ Normal

Merge Risk: ⚪ Minimal · up to dd963

The previously identified source-lint failure is fixed, and no remaining issue was established. This change is ready for normal merge checks.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title clearly and concisely describes the primary changes: relative-color folding and preserving expressions when folding is not exact.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Description check		The description provides a detailed explanation of the problem, fix, scope, limitations, performance impact, and verification results. It does not use the template headings exactly, but it includes th…

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Explanation

The description provides a detailed explanation of the problem, fix, scope, limitations, performance impact, and verification results. It does not use the template headings exactly, but it includes the required information, including how the changes were verified.

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Review comments at @src/css/properties/custom.rs:
- Line 1533: Replace the manual byte search in the surrounding function with
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• src/css/properties/custom.rs
• src/css/rules/property.rs
• src/css/values/angle.rs
• src/css/values/calc.rs
• src/css/values/color.rs
• src/css/values/time.rs
• src/css_jsc/color_js.rs
• test/bundler/css/wpt/relative_color_out_of_gamut.test.ts
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[robobun]

Two notes from #46221, which changes parse_hsl_hwb_components on main.

hwb(). #46221 removes the 100% cap of whiteness and blackness: from 100% in sum the color is the gray W / (W + B) (w3c/csswg-drafts#8444, WPT color-computed-hwb.html). One row that this PR pins then changes: hwb(from #336699 h 150% b) folds to #c9c9c9. Chrome 153 computes color(srgb 0.789474 0.789474 0.789474) for it, the same gray, inside sRGB. ComponentParser::clamp_unit here clamps every caller to 0..1, so after a rebase it needs no upper bound for hwb().

hsl() with no origin. The body lists hsl(210 150% 40%) under "Seen on main and not fixed here". A fix for main was in an earlier version of #46221 and is removed from it. With such a value left as written on main, color-mix(in srgb, hsl(210 150% 40%), rgb(0,0,255)50%) printed color-mix(in srgb,hsl(210 150% 40%),#00f50%), and a border-image moved after the border that resets it. This PR repairs that path, so the rule belongs on top of it: a saturation or lightness above 100% in the space-separated syntax fails the parse also for a color with no origin, unless the caller sets MUST_FOLD. The comma syntax keeps the clamp. That change is not built on this branch.

It needs one decision first, because browsers do not agree on this color:

• Chrome 153 (style sheet rule) and WebKit (source) keep the channel. Chrome computes color(srgb -0.2 0.4 1) inside a color-mix() and paints rgb(0, 102, 255) on an sRGB screen.
• Firefox clamps in every syntax and paints rgb(0, 102, 204), which is what Bun prints. Chrome's style.color setter and canvas give the same value.
• 25% of linear-gradient(to right, hsl(210 150% 40%), white) in Chrome 153 is (26, 140, 255) from the source, (64, 140, 216) from #06c and (64, 140, 255) from the clip #06f.

Question for a maintainer: does the order from the review of #33047 (pass the color through first) also apply to an hsl() with no origin above 100%, or does it keep the clamp, as in Firefox and lightningcss?

[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[Jarred-Sumner]

Part of this landed in #47025 (7942008). Its description says what was taken.

Relative colors: channel keywords are on the scale of their function (rgb(from #369 calc(r + 10) g b)), the origin's alpha is kept, a relative color kept as written gets no @supports copies, and a result outside sRGB is not folded. 31 real stylesheets

[robobun]

One part of this PR is not in #47025 and still reproduces on main 7942008: commit 3 (4ca1f11), the relative rgb() or hsl() whose unresolved alpha names a channel.

• MDN's guide "Using relative colors" teaches the input: rgb(from teal r g b / calc(alpha * var(--alpha-multiplier))). Main prints rgb(0 128 128/calc(alpha*var(--alpha-multiplier))). With --alpha-multiplier: 0.3, Chrome 153 computes color(srgb 0 0.501961 0.501961 / 0.3) from the source and rgba(0, 0, 0, 0) from that output.
• One public application stylesheet has it (LaplaceYoung/api_manager, src/styles/themes.css, 30 declarations). Of 10,831 stylesheets from 343 npm packages, none changes. There is no user report.

#47145 carries that commit alone, ported to main. #47025 does not say why it stayed out. If it was cut on purpose, close #47145.

#47145 also puts one choice to you. It keeps the fold and declines it when the alpha names a channel. The other way is never to fold a relative rgb() or hsl() with an unresolved alpha: 2 lines, and the same choice that is open on #31924. Both were built, and the PR compares them.

[robobun]

A second part of this PR is not in #47025: the FallbackHandler guard of commit 5 (60c98ab), which kept the earlier color or text-shadow before a relative color with no var().

#47284 carries it with a wider rule: any value that stays as tokens and has no var() or env() keeps the declaration before it. Its description lists the real stylesheets that have the input.