[robobun]

Problem

• round(-2.5px, 1px) folds to -3px and round(up, 23px, -10px) to 20px. CSS Values 4 and Chrome 153 give -2px and 30px.
• fn round (src/css/values/calc.rs:1370) rounds, floors and ceils the signed quotient. f32::round breaks ties away from zero. A negative step swaps up and down.

Fix

• fn round divides by |B| and picks the floor or the ceiling of that quotient. nearest takes the ceiling on a tie.
• A finite value with an infinite step gives the spec's zero or infinity.
• Correct because the spec picks by direction, not by the sign of B. Blink, WebKit and Gecko agree.
• Verified: test/js/bun/css/css.test.ts (63 new rows, 34 fail on main), test/js/bun/css/color.test.ts. Also ran test/js/bun/css/ and test/bundler/css/. Self-reviewed: 6 concerns raised, 6 addressed.

Background

• The CSS parser folds a math function whose arguments share a unit. Only fn round evaluates round().
• Considered (v + 0.5).floor() in the nearest arm only: round(15px, -10px) goes from 20px to 10px. lightningcss, the port source, has the old formula.

Downsides

• Folds that matched Chrome by accident turn wrong: 62 of 3,232 negative-step up/down rows on whole percents, 64 of 1,920 on relative colors, 140 of 456 negative decimal-step ties (2,426, 328 and 188 turn right). So does background-position: round(-75%, 50%) 0 with an image larger than its box.
• @media (min-width: calc(100px / round(-.5, 1))) now fails with "Invalid media query", as round(-.4, 1) does on main: Bun rejects a zero divisor.
• A folded round() runs 14 to 23 more instructions. .text shrinks by 1,792 B. Other CSS runs the same instructions.

Notes

Repro

printf '.a { margin-left: round(-2.5px, 1px); margin-top: round(-7.5px, 5px); margin-right: round(2.5px, 1px); margin-bottom: round(7.5px, 5px) }\n' > in.css
bun build ./in.css --outdir out && cat out/in.css

Main and 1.4.3 print margin: -10px 3px 10px -3px;. This branch prints margin: -5px 3px 10px -2px; (bundle, --minify and --no-bundle).

Reach

No issue reports this. It is a conformance fix found by reading the fold. Of 5,256 CSS files from npm packages (daisyui, vuetify, @fluentui/web-components, @wordpress/ui and others), 7 use round(). Each call has a var() or a relative color keyword as an operand, so none folds, and the 7 files build to the same output on main and here.

References

• CSS Values 4, "Stepped Value Functions": "lower B" is the multiple of B nearer to -∞ and "upper B" the multiple nearer to +∞. nearest takes the closer one, and upper B on a tie. up takes upper B and down takes lower B. "Argument Ranges" gives the results for a zero or an infinite argument.
• Blink GetNearestMultiples (math_functions.h), WebKit getNearestMultiples (CSSCalcExecutor.h) and Gecko (calc.rs, step.abs()) build the two candidates from |B|.
• Chrome 153.0.8010.52 computes the same value for the source and for the expected output of each new row (getComputedStyle).
• WPT css/css-values/round-function.html: the "negative step" block uses the default strategy only, and main passes it. The test keeps its six rows because the one-line variant breaks one of them (round(15px, -10px)). WPT has no two-argument negative tie and no up or down row with a negative step. Its rows for a finite value with an infinite step are the WPT rows that this PR changes.

What changes

This is a check that the old and the new body are equivalent outside the fix. It is not a measure of how many stylesheets change. Both bodies were compiled standalone and compared bit for bit (NaN equal to NaN). They differ in three classes only: nearest with A < 0 when the f32 quotient is a tie (n + 0.5), up or down with B < 0 on a value that is not a multiple, and a finite A with an infinite B.

• 127,999,968 synthetic samples (decimals with up to 3 places, common steps, a quarter with a negative step): 15,133,920 differ (211,132 ties, 14,922,788 negative steps). 0 of the 48,012,124 samples with A >= 0 and a finite B > 0 differ.
• 127,999,968 random bit patterns: 15,864,648 differ (57,142 ties, 15,807,506 negative steps).
• All 2^32 values of A with B = 1, four strategies: 8,388,608 differ, all negative ties.
• Against a double-precision transcription of the spec, on operands that f32 holds exactly: 64,080 grid rows, 15,000 wrong on main, 0 wrong here. 880 special-value rows (zeros, infinities, NaN): 222 wrong on main, 0 wrong here.

Operands that are not exact

Bun stores a percentage as an f32 fraction, a length as f32, and the origin of a relative color as 8-bit sRGB. A value that is a multiple or a tie on paper is then slightly off, and the fold goes to a neighbor. Main has the same noise. Its sign error hid it for a negative step, and its tie rule agreed with Chrome's double arithmetic on some decimal steps. "Right" below means that Chrome 153 computes the same value for Bun's output as for the source.

• Whole percents, up and down, negative step, 3,232 rows: 2,426 go from wrong to right, 744 stay right, 62 go from right to wrong. The 62 are exact multiples such as round(up, 9%, -1%) (9% to 10%). f32 holds 9% / 1% as 9.000001. Main prints the same 10% for round(up, 9%, 1%).
• Whole-percent ties, default strategy, positive step, 764 rows: 382 right on main, 677 right here. The other 87 are not ties in f32 (round(65%, 10%) prints 60% on main and here, Chrome 70%).
• hsl(from hsl(H S% 50%) round(strategy, h, step) s l), 1,920 rows: with a negative step and up or down, 328 go from wrong to right and 64 from right to wrong (hsl(from hsl(45 50% 50%) round(down, h, -15) s l) printed #bf9f40, now #bf8040, which main prints for a step of 15). The other rows do not change.
• Negative ties on a step that f32 cannot hold (0.1px to 2.9px), 456 rows: 188 go from wrong to right, 140 from right to wrong (round(-0.55px, 0.1px) printed -.6px, now -.5px, Chrome -0.6px), 75 stay right, 53 stay wrong. Bun decides the tie on the f32 quotient. Chrome decides it in doubles.
• A quotient of 2^22 or more: f32 holds it only in steps of 0.5, so a value that is not a multiple looks like a tie. round(-12582914, 3) folded to -12582915 (the nearer multiple) and now folds to -12582912. round(-12582913, 3) folded to -12582915 and now folds to -12582912 (the nearer multiple). Of 12,000 integers near 2^22 and 2^21 steps, main is wrong on 2,858 and this branch on 2,857. The printer keeps 6 significant digits, so the difference shows only inside a calc() that cancels the magnitude: calc(round(-12582914, 3) + 12582912) is -3 on main and 0 here. A quotient in f64 gets these rows right and breaks whole percents. In a standalone model of the fold, the rows that differ from exact arithmetic go from 337 to 1,742 of 19,200 (round(down, -299%, 1%) becomes -300%), because the f32 quotient absorbs the rounding error of operands such as 299%.

Percentage basis

Bun folds two percentages as if the value they resolve against were not negative. Main does the same for min(), max(), clamp(), rem(), mod() and hypot(). The basis of background-position is the box size minus the image size. With a 600px image in a 400px box, Chrome 153 paints round(-75%, 50%) 0 with the image edge at 200px. Main's -100% 0 paints it at 200px and this branch's -50% 0 at 100px. With a 400px image in a 600px box the source paints at -100px, main at -200px and this branch at -100px. Bun folds two-percentage round() in background-position, background-position-x and -y, mask-position-x and the background and mask shorthands. It leaves mask-position, object-position and offset-position as written.

Results that reach code this PR does not change

• A zero angle prints as rotate: none, as rotate: 0deg does on main (css: keep rotate: 0deg out of none, and keep the origin alpha in relative colors #38486 is open for that). rotate: round(-15deg, 30deg) printed -30deg and now prints none. Chrome computes 0deg.
• An infinite hue prints #000, as hsl(calc(infinity) 100% 50%) does on main. hsl(round(up, 45, infinity) 100% 50%) printed red (NaN printed as 0) and now prints #000. Chrome computes red.
• Bun does not fold a division by zero. In a declaration, calc(1px / round(-.5, 1)) printed -1px and now stays as written. In a media query it is an error (see Downsides). The same holds for a division by a round() with an infinite step: 28 of 32 such media queries now fail to parse.

Not fixed here

• rem() takes the sign of the divisor: rem(-18px, 5px) prints 2px, Chrome -3px. css: fold rem() with a truncated quotient, so it keeps the sign of the dividend #32286 is open for it.
• Mixed length units: round(1in, 10px) prints 96px, Chrome 100px. css: keep math functions over two different length units as written #38489 is open for it.
• mod() and rem() with an infinite divisor print 0px (Chrome: mod(5px, infinity) is 5px). mod(70%, 10%) prints 10% (Chrome 0%).
• Percent operands with a positive step: round(up, 30%, 1%) prints 31% on main and here (Chrome 30%).
• Relative colors: rgb(from rgb(10 20 30) round(r, 1) g b) prints #00141e on main and here (Chrome keeps r at 10, css: resolve relative color channel keywords as numbers in the function's range #38553 is open). hsl(from hsl(45 50% 50%) round(h, 30) s l) prints #bf8040 on main and here (Chrome computes a hue of 60).
• Bun does not parse the one-argument round(A). A declaration keeps it as written. @media, @container and @property fail to parse it, on main and here.

Measurements (two release builds of 80ae613, with and without this change, linux-x64, bun-profile. Main has one src/css change after that commit, #45210 in transform.rs. It does not touch the calc code.)

• .text: 80,193,381 B (main) vs 80,191,589 B (PR), delta -1,792 B (llvm-size -A).
• Round arm: 14,378 B in 11 bodies with 96 roundss (main) vs 12,471 B in 11 bodies plus 177 B calc::round with 3 roundss (PR). #[inline(never)] keeps one copy of the body. Main inlines it into each value type.
• Per folded round(), instructions inside Calc<LengthPercentage>::apply_op, entry to return: nearest 91 to 94 (was 71), up 89 (was 66), down 86 (was 67), to-zero 88 (was 67), infinite step 85 to 86 (was 66 to 71). That includes 1 direct call and return. No allocation is added.
• Without round(): 7 of 7 Calc<V>::parse_with bodies have the same size and the same instructions. 2,617 of 2,639 bun_css symbols have the same size, and the other 22 are the round arm. 30 of 30 fixture outputs are identical (the 15 files in test/js/bun/css/files, plain and --minify). margin-left: 2px runs 137 instructions in parse_with in both builds, min(13px, 10px) runs 5,679 in both.
• Instruction counts come from single-stepping with ptrace. valgrind and perf are not available in the build container, so there is no cachegrind number.

Self-review

The review asked for these changes. All are in this PR.

• A condition on the percent claims: they hold for a basis that is not negative ("Percentage basis").
• Rows for a value that is already a multiple, with up, down and to-zero.
• Rows that pin which zero an infinite step gives, the guards at a zero value, and NaN for two infinite arguments with up and down.
• The two rows that no change could break: one now uses up, the other sits with the exact multiples.
• The negative ties on a decimal step that stop matching Chrome ("Operands that are not exact").
• The list of position properties that fold.

Not taken: a line on the merge order with #45084 and #39514. Their diffs do not show that they send more inputs to fn round.

Suites

• bun bd test test/js/bun/css/css.test.ts test/js/bun/css/color.test.ts: 2,403 pass, 0 fail. A debug build of 80ae613 fails 37 (34 of the 63 new rows in css.test.ts, 3 of the 4 in color.test.ts).
• The two release builds, test/js/bun/css/, test/bundler/css/ and test/bundler/esbuild/css.test.ts (30 files): 37 fail without the change (the new rows), 0 fail with it.
• bun bd test test/js/bun/css/ (debug build, 20 files): the only failures are timeouts in css-fuzz, nested-selector-expansion, nested-selector-list-expansion and nested-vendor-prefix-duplication. A debug build without the change times out in the same tests on the same machine.

[robobun]

^{Updated 8:51 AM PT - Oct 14th, 2026}

✅ @robobun, your commit 3bd1cb84462b4309441bb1cb025d82b190bb1d4c passed in Build #125486! 🎉

---

🧪   To try this PR locally:

bunx bun-pr 45249

That installs a local version of the PR into your bun-45249 executable, so you can run:

bun-45249 --bun

[robobun]

Status

Closed. The tie rule and the negative step landed in #47025 (7942008).

Reproduced before the fix on 80ae613 and on 1.4.3:

printf '.a { margin-left: round(-2.5px, 1px); margin-top: round(-7.5px, 5px); margin-right: round(2.5px, 1px); margin-bottom: round(7.5px, 5px) }\n' > in.css
bun build ./in.css --outdir out && cat out/in.css

It printed margin: -10px 3px 10px -3px;. Main now prints margin: -5px 3px 10px -2px;, which is what Chrome 153 computes.

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Walkthrough

The CSS round() helper now uses the absolute interval and rounds nearest ties upward. Tests cover numeric edge cases, CSS minification contexts, and color channels.

Changes

CSS round() behavior

Layer / File(s)	Summary
Rounding calculation and coverage src/css/values/calc.rs, test/js/bun/css/css.test.ts, test/js/bun/css/color.test.ts	The helper uses the absolute interval, applies the selected rounding strategy, and handles infinite intervals. Tests cover ties, signed zero, non-finite values, CSS folding contexts, and color channels.

Suggested reviewers: jarred-sumner

Priority: ➖ Normal

Merge Risk: 🔵 Low · up to 3bd1c

Some CSS values can be rounded to the wrong multiple during folding. The affected precision edge case is narrow, but should be corrected or explicitly accepted before relying on the folded output.

Pre-merge checks: 4

Passed checks: 4 passed

Check name	Status	Explanation
Title check		The title clearly summarizes the main change: CSS round() folding now selects between lower and upper multiples of the absolute step.
Description check		The description explains the problem, fix, scope, behavior changes, limitations, performance impact, and verification results. It does not use the exact template headings, but it provides the required…
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.

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Actionable comments posted: 1

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Inline comments:
Review comments at @src/css/values/calc.rs:
- Line 1390: Update the nearest-multiple rounding logic around the quotient and
lower-candidate comparison to use higher-precision intermediates when selecting
the multiple, converting to f32 only afterward. Preserve CSS’s distance-based
choice and use the upper multiple only for an actual tie; add a regression case
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[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.

[claude]

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

Beyond the inline rem() finding, I also checked the sibling mod() arm at src/css/values/calc.rs:407-420 for the same sign-class bug — it is floored modulo (sign of divisor), which is what CSS Values 4 specifies for mod(), so it is not affected. I also walked the new infinite-step branch and the nearest tie comparison against the Values 4 argument-range table (up with +0, down with -0, NaN step, NaN value) and each case matches the spec.

Extended reasoning...

The change rewrites the round() constant-folding helper in src/css/values/calc.rs to pick between the lower and upper multiple of |step| with ties toward +infinity, plus explicit handling of a finite value with an infinite step, and adds ~70 minify_test rows and 4 color-channel rows. It touches no security-sensitive surface. The only new commit since the last review trims a doc comment to one line; the posted finding is a pre-existing sibling bug in rem(), and a further verified finding was not posted, so this is not an approval.

[Jarred-Sumner]

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

round() breaks ties upward and handles a negative step as browsers do

[robobun]

The repro prints margin: -5px 3px 10px -2px; on main (7942008), so the ties and the negative step are fixed there.

For the record, the body that landed ((v + 0.5).floor()) and the body of this PR differ in four narrow cases. I checked them on a debug build of main.

• round(0.49999997px, 1px) prints 1px. 0.49999997 is the largest f32 below 0.5, and v + 0.5 rounds to 1.0. The nearer multiple is 0.
• An odd quotient of 2^23 or more: v + 0.5 rounds to the even neighbor, so a value that is already a multiple moves one step. round(8388609px, 1px) prints 8388610px, and round(-8388609px, 1px) prints -8388608px.
• A zero result is +0 where the spec gives -0. calc(1px * pow(round(-.4, 1), -1)) prints 3.40282e38px. Chrome 153 computes the most negative length, and main before css: consolidate the open CSS parser fixes #47025 printed -3.40282e38px.
• A finite value with an infinite step is still NaN: round(up, 5px, calc(infinity * 1px)) prints 0px.

Over all finite f32 values with a step of 1, the landed body differs from exact arithmetic on 8,388,609 values, and the body of this PR on none (both compiled standalone). The tie test of this PR, v - v.floor() < 0.5 with v.ceil() as the other branch, does not have the first three cases.