[robobun]

Problem

• The "Math functions" example in docs/bundler/css.mdx shows --scale-factor: pow(1.25, 3) printed as --scale-factor: 1.953125, and transform: rotate(35.36deg). bun build prints --scale-factor: pow(1.25, 3) and rotate(35.3553deg), in a different order.
• The page says the bundler evaluates math "when all values are known constants". It does not say that this applies only to properties whose values Bun parses.

Fix

• The output block is now the output of bun build for the example, on 1.4.3 and on main 7942008688.
• The paragraph names the two conditions. The constants are in units that Bun can combine, so clamp(200px, 50%, 800px) stays as written. Bun parses the value of the property, so custom properties and all other values stay as written.
• Verified: a script takes the two code blocks out of the page, builds the first, and compares the result with the second.

Background

• Bun parses the values of 251 properties into typed values (width, padding, transform, and more). The parser evaluates a constant math function there.
• Bun keeps each other value, and each custom property, as a list of tokens. Nothing evaluates a function in a list of tokens. lightningcss, which Bun's CSS code is a port of, does the same.
• Considered: evaluate math inside custom properties, which makes the old example true. Rejected, because a list of tokens does not know the type of the slot. z-index: calc(5 / 2) is valid and rounds to 3. The literal 2.5 is invalid there.

Notes

• The check, run from an empty directory with DOC set to the path of docs/bundler/css.mdx. It prints nothing when the page and the build agree:

  awk '/^### Math functions/{s=1; next} /^### Media query ranges/{s=0} s && /^```css/{n++; b=1; next} s && /^```$/{b=0; next} s && b && n==1{print > "styles.css"} s && b && n==2{print > "expected.css"}' "$DOC"
  bun build ./styles.css --outdir out > /dev/null
  grep -v '^/\* styles.css \*/$' out/styles.css | sed '/^$/d' | diff - expected.css

• Reference outputs for the same example: lightningcss 1.33.0 also keeps --scale-factor: pow(1.25, 3). esbuild 0.28.2 evaluates calc() only, and only with --minify.

• css: consolidate the open CSS parser fixes #47025 consolidated the open CSS parser pull requests. Docs-only pull requests were out of its scope, and it did not change this section. The check above prints nothing on main 7942008688, for this branch merged into it.

• After css: consolidate the open CSS parser fixes #47025, round(), mod(), rem() and hypot() over two different length units stay as written (round(1in, 10px)). The first condition in the new paragraph covers that case: those are units that Bun does not combine.

• text-shadow is a property that Bun parses and that keeps math as written, because its lengths are read without a math arm. css: parse math functions in text-shadow lengths #45546 proposed a typed parse for it and was not taken.

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no test proof · iteration 0 · docs-only change; test-proof not applicable

[robobun]

^{Updated 11:29 PM PT - Oct 12th, 2026}

✅ @robobun, your commit d7f554a5f86ac5c3372eb4deeeaa62f3bfbdde07 passed in Build #125019! 🎉

---

🧪   To try this PR locally:

bunx bun-pr 45114

That installs a local version of the PR into your bun-45114 executable, so you can run:

bun-45114 --bun

[robobun]

Status: ready for review.

• Reproduced: built the example from the "Math functions" section with Bun 1.4.3 and with main 7942008688 (after css: consolidate the open CSS parser fixes #47025). Both print --scale-factor: pow(1.25, 3) and transform: rotate(35.3553deg). The page shows 1.953125 and 35.36deg.
• This PR changes docs/bundler/css.mdx only. The output block now matches the build.
• css: consolidate the open CSS parser fixes #47025 did not change this section, and this branch merges into main without a conflict.

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Walkthrough

The CSS bundler documentation now explains when Bun evaluates math expressions at build time. The example preserves expressions in custom properties and unparsed values, and shows evaluated values for padding and transform.

Changes

CSS Math-Function Documentation

Layer / File(s)	Summary
Math-function evaluation explanation and example docs/bundler/css.mdx	The documentation limits evaluation to constants with combinable units in parsed properties. The example preserves pow() and var() expressions and shows padding as 15px and transform as 35.3553deg.

Priority: Low

Merge Risk: Minimal · up to d7f55

The documentation now describes when compatible math in parsed CSS values is reduced and when expressions remain for the browser. No actionable merge risk remains.

Pre-merge checks: 4

Check name	Status	Explanation
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.
Title check		The title clearly identifies the documentation correction and the clarification of where CSS math is evaluated.
Description check		The description explains the problem, the documentation changes, the verification method, and relevant technical background. It does not use the exact template headings, but it provides the required i…

Full details: Description check

Explanation

The description explains the problem, the documentation changes, the verification method, and relevant technical background. It does not use the exact template headings, but it provides the required information and is complete.

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @docs/bundler/css.mdx:
- Line 616: Revise the CSS bundler explanation to state that build-time math
evaluation requires Bun to parse the property value and reduce its known
operands, including compatibility between typed operands. Preserve the existing
explanation of custom properties, unparsed values, and browser evaluation.

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[claude]

Code review found no issues

No high-confidence issues detected in this change.

[claude]

Code review found no issues

No high-confidence issues detected in this change.