[robobun]

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 Vite's baseline widely available set #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 css: print a stylesheet with the targets StyleSheet::minify recorded #39251 and bake: minify stylesheets for browser targets in every graph #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

[robobun]

^{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]

Status

Landed on main in #47025 (7942008). This PR is closed.

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 the change it prints color: light-dark(#102030, #d0e0f0);.

Checked on main at 7942008:

• src/css/targets.rs has the LIGHT_DARK bit, and Targets::browser_default() excludes it. The gates in src/css/values/color.rs, src/css/properties/custom.rs and src/css/properties/ui.rs use should_compile_same(Feature::LightDark).
• The three default-target cases are in test/js/bun/css/css.test.ts, under light-dark() under the default targets of the bundler.
• No CSS test failed in build 133627, the CI run of the head of css: consolidate the open CSS parser fixes #47025. The CSS sources and tests of that commit are identical to main.

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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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Actionable comments posted: 1

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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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• src/css/properties/custom.rs
• src/css/properties/ui.rs
• src/css/targets.rs
• src/css/values/color.rs
• test/bundler/css/light-dark-default-targets.test.ts

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[robobun]

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.

[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.

[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…

If you have decided not to act on one of these findings, resolve its thread (a reply alone leaves it open) and the next review stops counting it. To review this commit again now, use Re-run on its "Claude Code Review" check.

[Jarred-Sumner]

Landed in #47025 (7942008).

Keep light-dark() as written under the default browser targets. 227 real stylesheets

[robobun]

Confirmed on main at 7942008. The LIGHT_DARK bit, the default exclude in Targets::browser_default(), the three should_compile_same gates, the docs change and the three default-target tests (now in test/js/bun/css/css.test.ts) are all there. Nothing from this PR is left out. No CSS test failed in the CI run of the head of #47025 (build 133627), and the CSS sources of that commit are identical to main.

Open PRs #39251 and #37051 still assert the old default output in their tests (--buncss-light, no light-dark(). They need new assertions when they rebase.