css: keep light-dark() as written under the default browser targets by robobun · Pull Request #42009 · oven-sh/bun

robobun · 2026-09-08T14:53:52Z

Problem

bun build rewrites light-dark(a, b) into var(--buncss-light, a) var(--buncss-dark, b). Only color-scheme rules in the same compiled CSS define those variables. With the scheme from <meta name="color-scheme">, another stylesheet, or nowhere, the value is a b: invalid (Color-scheme rules inside @layer do not inject --buncss-light / --buncss-dark variables #20689).
Cause: the gates (src/css/values/color.rs:511, properties/custom.rs:946, properties/ui.rs:120) compare raw browser versions. The default floor (src/css/targets.rs:190, chrome 80) is below chrome 123.

Fix

Add lightningcss's LIGHT_DARK bit to targets::Features, route the gates through should_compile_same, and exclude the bit in Targets::browser_default(). Default builds print light-dark() and color-scheme as written. Explicit targets still polyfill.
It reverses documented output (fix(css): process color-scheme rules inside @layer blocks #24034): needs a maintainer yes. Self-reviewed: 7 concerns raised, 4 distinct, 3 addressed. Rejected: plumbing only, which changes no output.
Verified: bundler default targets in test/js/bun/css/css.test.ts (3 cases, all fail on 1.4.2) and test/bundler/css/.

Background

should_compile lowers a feature when include has its bit, or exclude lacks it and a target lacks support.
Considered lowering only when the bundle compiles a color-scheme rule: wrong when that rule misses the element. Considered printing polyfill then native: custom properties need an @supports copy, and each declaration doubles.

Downsides

Browsers between the default floor and light-dark() support hold 5.0% of global usage (caniuse-lite 1.0.30001810). On pages that compile color-scheme on an ancestor they had the right colors and now drop each light-dark() declaration. No opt-in until Bun.build has no way to set CSS browser targets, so oklch() is always downlevelled (minified output larger than input) #40361.
Speed, size: none found. Only light-dark() and color-scheme reach the gates. The docs example minifies to 216 bytes (was 492).

Notes

Repro: .b { color: light-dark(#102030, #d0e0f0) } with bun build ./entry.css on 1.4.3 prints color: var(--buncss-light, #102030) var(--buncss-dark, #d0e0f0);. In Chromium with no color-scheme rule the computed color is the inherited one in light and dark mode. After this change it prints color: light-dark(#102030, #d0e0f0);.
How the polyfill works: it adds --buncss-light: initial; --buncss-dark: ; to each color-scheme rule that names light or dark, swapped under prefers-color-scheme: dark. A var() of an initial or unset variable takes its fallback, and an empty one vanishes. With both unset, both fallbacks print.
The 5.0% in Downsides is the sum of browserslist coverage for chrome 80 to 122 (3.03%), edge 80 to 122 (0.41%), firefox 78 to 119 (0.13%), safari 14 to 17.4 (0.36%), ios_saf 14 to 17.4 (0.83%), opera 67 to 108 (0.18%) and samsung 13 to 26 (0.07%). All browsers with light-dark() cover 88.76% of 96.69% tracked.
The rejected design "lower only when the bundle compiles a color-scheme rule", checked in Chrome 153: .dark-theme { color-scheme: dark } .card { color: light-dark(#102030, #d0e0f0) }. A .card outside .dark-theme computes to rgb(16, 32, 48) from the source and from this branch, and to the inherited rgb(0, 0, 0) from the polyfilled output. The rule would also make the output of one file depend on the declarations of another file in the bundle.
The rejected design "print the polyfill, then the native declaration": modern browsers take the native one and old browsers keep the polyfill. A custom property accepts any value at parse time, so --x: light-dark() always wins in old browsers and needs an @supports (color: light-dark(red, red)) copy of the rule. Every light-dark() declaration doubles in size. This is the design to build if browsers below light-dark() support must keep the polyfill by default.
css: bump the default browser targets to baseline widely available #40369 (default floor to chrome 107) stays below chrome 123, so it does not change this on its own.
With a future targets option (Bun.build: add cssTarget option to set CSS browser targets #40368), a user who asks for targets below light-dark() support gets the polyfill, the same as lightningcss with those targets. The docs paragraph describes the lowering and says the default targets leave it out, so it stays true either way.
light-dark(oklch(), oklch()) still gets the rgb, p3 and lab fallback tiers, each inside light-dark(). lightningcss with exclude: LightDark and the same targets prints the same.
The existing polyfill tests in test/js/bun/css/css.test.ts pass explicit chrome 90 targets through the internal test API, which does not set exclude, so they are unchanged. The new cases sit in the same color-scheme block and go through Bun.build, the only path that uses the default targets. Open PRs bundler: print each stylesheet with the CSS targets it was minified for #39251 and bake: apply browser CSS targets to stylesheets imported on the server #37051 assert default-target polyfill output in bake fixtures and would need a fixture update if this lands first.
src/css/build-prefixes.js regenerates the Features block, so the bit is added to its list too. COLORS includes it, as in lightningcss.
Bun has no user-facing CSS targets option yet (Bun.build has no way to set CSS browser targets, so oklch() is always downlevelled (minified output larger than input) #40361, Allow configuring or disabling CSS syntax lowering #40133).
Also ran test/bundler/esbuild/css.test.ts, test/bundler/bundler_html.test.ts, test/js/bun/css/color.test.ts, css-loader.test.ts, duplicate-declaration-merge-hang.test.ts, token-list-backtracking.test.ts, test/regression/issue/css-system-color-contexts.test.ts: all pass, on the branch merged with main at 6d504dd.

[human-review] gate passed · iteration 3 · 7 files touched

fails on main (without fix)

ASAN without fix: 3 failed, 69 skipped
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/pr_gate.xml" test/js/bun/css/css.test.ts
bun test v1.4.3 (367d939d9)

test/js/bun/css/css.test.ts:
Output .flexrow {
  flex-direction: row;
}

.flexcol {
  flex-direction: column;
}

.hello {
  flex-wrap: wrap;
}

.world {
  flex-wrap: nowrap;
}

(pass) css tests > .flexrow {
	flex-direction: row;
}

.flexcol {
	flex-direction: column;
}

.hello {
	flex-wrap: wrap;
}

.world {
	flex-wrap: nowrap;
} [3.26ms]
Output .foo {
  content: "+";
}

(pass) css tests > .foo {
  content: "\2b";
} [1.05ms]
Output div {
  --foo: 1 1 1 #0101011a, 2 2 2 #02020233;
  --bar: 1 1 1 #01010116, 2 2 2 #02020233;
}

(pass) css tests > custom property cases > div {
        --foo: 1 1 1 rgba(1, 1, 1, 0.1), 2 2 2 rgba(2, 2, 2, 0.2);
        --bar: 1 1 1 #01010116, 2 2 2 #02020233;
      } [0.97ms]
Output :root {
  --my-color: red;
  --my-bg: white;
  --my-font-size: 16px;
}

.element {
  color: var(--my-color);
  background-color: var(--my-bg);
  font-size: var(--my-font-size);
}

(pass) css tests > custom property cases > :root {
        --my-color: red;
        --my-bg: wh
... (truncated)

release without fix: 69 skipped
bun test v1.4.3-canary.1 (faddb541b)

test/js/bun/css/css.test.ts:
Output .flexrow {
  flex-direction: row;
}

.flexcol {
  flex-direction: column;
}

.hello {
  flex-wrap: wrap;
}

.world {
  flex-wrap: nowrap;
}

(pass) css tests > .flexrow {
	flex-direction: row;
}

.flexcol {
	flex-direction: column;
}

.hello {
	flex-wrap: wrap;
}

.world {
	flex-wrap: nowrap;
} [0.15ms]
Output .foo {
  content: "+";
}

(pass) css tests > .foo {
  content: "\2b";
} [0.04ms]
Output div {
  --foo: 1 1 1 #0101011a, 2 2 2 #02020233;
  --bar: 1 1 1 #01010116, 2 2 2 #02020233;
}

(pass) css tests > custom property cases > div {
        --foo: 1 1 1 rgba(1, 1, 1, 0.1), 2 2 2 rgba(2, 2, 2, 0.2);
        --bar: 1 1 1 #01010116, 2 2 2 #02020233;
      } [0.05ms]
Output :root {
  --my-color: red;
  --my-bg: white;
  --my-font-size: 16px;
}

.element {
  color: var(--my-color);
  background-color: var(--my-bg);
  font-size: var(--my-font-size);
}

(pass) css tests > custom property cases > :root {
        --my-color: red;
        --my-bg: white;
        --my-font-size: 16px;
      }
      .element {
        color: var(--my-color);
        background-color: var(--my-bg);
        font-size: 
... (truncated)

passes on PR (with fix)

ASAN with fix: 69 skipped
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/pr_gate.xml" test/js/bun/css/css.test.ts
bun test v1.4.3 (367d939d9)

test/js/bun/css/css.test.ts:
Output .flexrow {
  flex-direction: row;
}

.flexcol {
  flex-direction: column;
}

.hello {
  flex-wrap: wrap;
}

.world {
  flex-wrap: nowrap;
}

(pass) css tests > .flexrow {
	flex-direction: row;
}

.flexcol {
	flex-direction: column;
}

.hello {
	flex-wrap: wrap;
}

.world {
	flex-wrap: nowrap;
} [2.60ms]
Output .foo {
  content: "+";
}

(pass) css tests > .foo {
  content: "\2b";
} [1.03ms]
Output div {
  --foo: 1 1 1 #0101011a, 2 2 2 #02020233;
  --bar: 1 1 1 #01010116, 2 2 2 #02020233;
}

(pass) css tests > custom property cases > div {
        --foo: 1 1 1 rgba(1, 1, 1, 0.1), 2 2 2 rgba(2, 2, 2, 0.2);
        --bar: 1 1 1 #01010116, 2 2 2 #02020233;
      } [0.94ms]
Output :root {
  --my-color: red;
  --my-bg: white;
  --my-font-size: 16px;
}

.element {
  color: var(--my-color);
  background-color: var(--my-bg);
  font-size: var(--my-font-size);
}

(pass) css tests > custom property cases > :root {
        --my-color: red;
        --my-bg: wh
... (truncated)

release with fix: 69 skipped
$ bun scripts/build.ts --profile=release
[configured] bun-profile → bun (stripped) in 766ms (unchanged)
ninja: Entering directory `/workspace/bun/build/release'
[1/24] rustc bun_css 
[2/24] rustc bun_bundler 
[3/24] rustc bun_transpiler 
[4/24] rustc bun_standalone_graph 
[5/24] rustc bun_bunfig 
[6/24] rustc bun_install 
[7/24] rustc bun_jsc 
[8/24] rustc bun_ast_jsc 
[9/24] rustc bun_sys_jsc 
[10/24] rustc bun_bundler_jsc 
[11/24] rustc bun_patch_jsc 
[12/24] rustc bun_semver_jsc 
[13/24] rustc bun_css_jsc 
[14/24] rustc bun_js_parser_jsc 
[15/24] rustc bun_sourcemap_jsc 
[16/24] rustc bun_install_jsc 
[17/24] rustc bun_http_jsc 
[18/24] rustc bun_sql_jsc 
[19/24] rustc bun_runtime 
[20/24] link bun-profile
ld.lld: warning: Linking two modules of different target triples: 'obj/vendor/mimalloc/src/static.c.o' is 'x86_64-pc-linux-gnu' whereas '../../../../root/.bun/build-cache/webkit-299c5323879e79af-lto/lib/libJavaScriptCore.a(UnifiedSource-runtime-31.cpp.o at 175447362)' is 'x86_64-unknown-linux-gnu'


ld.lld: warning: Linking two modules of different target triples: 'obj/codegen/GeneratedSSLConfig.cpp.o' is 'x86_64-pc-linux-gnu' whereas 'rust-target/x86_64-unkn
... (truncated)

diff hotspot

docs/bundler/css.mdx         | 37 ++-------------------------
 src/css/build-prefixes.js    | 11 +++++++-
 src/css/properties/custom.rs |  5 +++-
 src/css/properties/ui.rs     |  4 +--
 src/css/targets.rs           |  7 ++++-
 src/css/values/color.rs      |  2 +-
 test/js/bun/css/css.test.ts  | 61 +++++++++++++++++++++++++++++++++++++++++++-
 7 files changed, 85 insertions(+), 42 deletions(-)

gate history · 3 passed · 0 rejected · iteration 3

evidence per changed file

file                          reads  edits  tests
docs/bundler/css.mdx              2      4     17
src/css/build-prefixes.js         1      1     17
src/css/properties/custom.rs      1      1     17
src/css/properties/ui.rs          1      1     17
src/css/targets.rs                3      3     17
src/css/values/color.rs           1      1     17
test/js/bun/css/css.test.ts       1      1     16

The light-dark() polyfill rewrites each call into var(--buncss-light, a) var(--buncss-dark, b). Those variables are only defined next to a color-scheme declaration compiled in the same stylesheet. When the page sets its scheme with <meta name=color-scheme>, in another stylesheet, in an inline style, or not at all, both variables are unset and the value is invalid at computed-value time. bun build applies browser targets on the user's behalf, and the default floor (chrome 80) is below light-dark() support (chrome 123), so every default build got the rewrite. Add the LIGHT_DARK feature bit that lightningcss gates this lowering with, route the three call sites through should_compile, and exclude the bit from the bundler's default targets. Explicit targets still produce the polyfill.

robobun · 2026-09-08T14:54:06Z

^{Updated 5:02 AM PT - Sep 23rd, 2026}

✅ @robobun, your commit 9ee9f5481730ea71d872e3de477bb93d9b3ebbb2 passed in Build #119982! 🎉

🧪 To try this PR locally:

bunx bun-pr 42009

That installs a local version of the PR into your bun-42009 executable, so you can run:

bun-42009 --bun

robobun · 2026-09-08T14:54:28Z

Status

Reproduced on bun 1.4.3 with the stylesheet from the report:

.b { color: light-dark(#102030, #d0e0f0); background-color: light-dark(white, black) }
.c { accent-color: light-dark(red, blue) }

bun build ./entry.css printed color: var(--buncss-light, #102030) var(--buncss-dark, #d0e0f0); with neither variable defined anywhere in the output. With this branch it prints color: light-dark(#102030, #d0e0f0);.

The bundler default targets cases in test/js/bun/css/css.test.ts (the color-scheme block) fail 3 of 3 on a stock bun 1.4.2 and pass on this branch.

The branch includes main at 6d504dd (clean merge). On the merged tree test/js/bun/css/css.test.ts, test/bundler/css/, test/bundler/esbuild/css.test.ts and test/bundler/bundler_html.test.ts pass. The first CI run (build 113001) had one red job, test/js/node/test/parallel/test-crypto-dh-leak.js on debian x64-asan. That test was red on main at the time and does not touch CSS.

Decision needed from a maintainer: this changes the default output for light-dark() and color-scheme (no variable rewrite unless targets are requested). The cost is in the Downsides section of the description: browsers between the default floor and light-dark() support (5.0% of global usage) lose the polyfill on pages that compile color-scheme on an ancestor. The alternative is to land only the LIGHT_DARK feature-bit plumbing and keep the current default until a CSS targets option exists (#40368, #40133).

coderabbitai · 2026-09-08T14:59:39Z

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test/js/bun/css/css.test.ts

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Walkthrough

The bundler’s default browser targets now preserve light-dark() and color-scheme. CSS serialization uses the target compilation policy to select native output or fallback variables. Tests and documentation cover this behavior and describe lowering limitations.

Changes

light-dark compilation behavior

Layer / File(s)	Summary
Target feature policy `src/css/targets.rs`, `src/css/build-prefixes.js`	Adds `LIGHT_DARK` to the target feature model and generated color feature group. Default browser targets exclude its compilation.
Serialization and property handling `src/css/values/color.rs`, `src/css/properties/custom.rs`, `src/css/properties/ui.rs`	Uses `should_compile_same(Feature::LightDark)` to select native `light-dark()` output or fallback variables.
Bundler validation and documentation `test/js/bun/css/css.test.ts`, `docs/bundler/css.mdx`	Tests that default-target builds preserve `light-dark()` and `color-scheme`. Documentation describes used color schemes and lowering limitations.

Suggested reviewers: jarred-sumner

Priority: ➖ Normal

Merge Risk: ⚪ Minimal · up to 9ee9f

Default builds preserve native light-dark() and color-scheme output, while explicitly targeted older browsers retain fallback compilation. No issue identified here needs resolution before merge.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes the main change: preserving light-dark() under default browser targets.
Description check	✅ Passed	The description explains the problem, implementation, behavior changes, trade-offs, documentation impact, and verification results. It does not use the exact template headings, but it provides the req…

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Inline comments:
In `@docs/bundler/css.mdx`:
- Line 306: Update ColorSchemeHandler::handle_property so color-scheme: normal
also emits the required light/dark variables, while preserving the existing
behavior for declarations containing light or dark. Ensure the variables are
available when the declaration applies to the same element or an ancestor.

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docs/bundler/css.mdx
src/css/build-prefixes.js
src/css/properties/custom.rs
src/css/properties/ui.rs
src/css/targets.rs
src/css/values/color.rs
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…iables

robobun · 2026-09-08T15:03:48Z

Addressed the two review notes: the docs sentence now says which color-scheme declarations define the variables and that they apply to the element or an ancestor (91926be), and the comment in Targets::browser_default() is one line (83f31dc). No code behavior changed since e60e8d3.

claude

Code review found no issues

No high-confidence issues detected in this change.

…light-dark-default-passthrough

claude

Beyond the inline findings, I also checked that the hand-edited Features block in src/css/targets.rs matches what src/css/build-prefixes.js would regenerate (light_dark is the 21st scalar flag, so bit 20, and it is in the colors group in both) and that the remaining direct Feature::LightDark.is_compatible(browsers) call at src/css/values/color.rs:546 is the property-level fallback check rather than the polyfill gate, so it is unaffected by the exclude bit.

Extended reasoning...

The change adds a LIGHT_DARK feature bit to the CSS targets bitflags, routes the three light-dark() polyfill gates through should_compile_same, and excludes the bit under Targets::browser_default(), which is also used for the BunMacro and ServerComponentsSsr targets; it touches no security-sensitive surface. The generator and hand-edited bitflags block were compared and agree, and the one remaining direct is_compatible call in color.rs serves a different purpose. The behavioral default change for older browsers is a design call flagged inline and still needs a maintainer decision.

Findings marked 🟡 are optional suggestions and need no follow-up push.

They sit next to the explicit-target light-dark() and color-scheme cases.

claude

Code review found no issues

No high-confidence issues detected in this change.

Still open from earlier reviews (1):

🔴 src/css/targets.rs:24 — Sites that declare color-scheme in the bundled CSS lose every light-dark() color in Safari 17.4-, Chrome 122- and Firef…

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robobun assigned Jarred-Sumner Sep 8, 2026

github-actions Bot added the claude label Sep 8, 2026

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Comment thread docs/bundler/css.mdx Outdated

docs: say which color-scheme declarations define the light-dark() var…

91926be

…iables

github-actions Bot reviewed Sep 8, 2026

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Comment thread src/css/targets.rs Outdated

css: shorten the default-targets comment

83f31dc

claude Bot reviewed Sep 8, 2026

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robobun mentioned this pull request Sep 19, 2026

css: skip fallbacks for features the enclosing @supports tests for #43421

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Merge remote-tracking branch 'origin/main' into robobun/dd8ddb30/css-…

faddb54

…light-dark-default-passthrough

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Comment thread src/css/targets.rs

Comment thread test/bundler/css/light-dark-default-targets.test.ts Outdated

test: move the light-dark() default-target cases into css.test.ts

9ee9f54

They sit next to the explicit-target light-dark() and color-scheme cases.

claude Bot reviewed Sep 23, 2026

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css: keep light-dark() as written under the default browser targets - #42009

css: keep light-dark() as written under the default browser targets#42009
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