css: keep same-condition @supports rules and empty named @layer blocks by robobun · Pull Request #46733 · oven-sh/bun

robobun · 2026-10-24T10:32:08Z

Fixes #24770

Part of #45258

Problem

CssRuleList::minify (src/css/rules/mod.rs:528) deletes the second of two adjacent @supports rules with one condition, with or without --minify. The five blocks of the issue build to one.
It deletes an empty named @layer a {} (:543), which fixes the layer's place in the cascade order: @layer a{}@layer b{}@layer b{x{color:red}}@layer a{x{color:blue}} builds to @layer b{x{color:red}}@layer a{x{color:#00f}}. Regression in 1.3.2 (fix(css): process color-scheme rules inside @layer blocks #24034).

Fix

The Supports arm keeps a rule whose condition equals the rule before it.
The LayerBlock arm drops only an empty anonymous @layer {}. A kept @supports (c){@layer a{}} makes the old arm delete the next rule, so both go together.
Verified: test/js/bun/css/css.test.ts (50 of 52 new cases fail on 1.4.4), five itBundled cases, test/js/bun/css/, test/bundler/css/.
Self-reviewed: 31 concerns raised, 24 addressed (Notes).

Background

@layer a {} and @layer a; both declare a. Browsers ignore the statement inside a style rule, so the block stays (supersedes css: keep empty named @layer blocks as @layer statements #38480).
Considered a merge into one rule (lightningcss). When the @media arm minifies a merged @media rule again, the duplicate-rule check deletes an !important or custom-property rule from the merged list (css: compare custom property names in PropertyId equality and importance in duplicate-rule detection #33684).

Downsides

Each @supports rule repeats its condition: the issue's input is 334 B, 234 B if merged.
A file of only empty named layer blocks now prints. Ahead of @namespace, Chrome drops the namespace rule. css: emit @layer statements that precede @import before the imported files #41572's model: 3 of 1,000 graphs regress.
Size and speed: none found. CssRuleList::minify is 48 B smaller.

Notes

Builds compared. Release builds of main 179664e and of this branch. Chrome 153 for the browser checks.

Same-condition @supports, bun build --minify, default targets

Input	main	this PR
The five blocks of #24770	1 of 5 declarations	5 of 5, 334 B
`.a { --x: 1; @supports (color: red) { --x: 2; } --y: 1; @supports (color: red) { --y: 2; } }` (#45258)	`--x:2` only	`--x:2` and `--y:2`
`.a{@supports (color: red){color:red!important}@supports (color: red){color:blue}}`	`color:red!important`	both rules, red wins
The same rule in an `@media` rule, with an equal `@media` rule after it	`color:red!important`	both rules, red wins
256 rules nested under `.a,.b`	exit 0, 1 of 256 rules	exit 0, 256 rules
8,000 adjacent rules, distinct declarations	34 ms, 1 rule	47 ms, 8,000 rules
16 rules with `var(--x, lab(50% 20 30))`	245 B, 1 rule	3,923 B, 16 rules

The other half of #45258 (:root { & { --a: 2 } & { --b: 2 } }) has no @supports. It needs #33684.

Why the rules are not merged. Two merge shapes were built and measured on this branch.

Append, then minify the merged rule again. That is the @media arm on main and the arm of lightningcss 1.33.0 (upstream changed it after that release, in perf: improve minification of adjacent conditional rules parcel-bundler/lightningcss#1263). The !important input prints only .a{color:#00f}, the CSS bundler drops nested @supports blocks and same-selector nested rules when lowering nesting #45258 input keeps --y:2 only, 256 nested rules fail with Nested CSS rules expand to more than 65536 selectors, 8,000 rules take 16.6 s, and the 16 lab() rules print 77,063 B.
Minify the incoming rule, then append, with no second minify. This is right for the inputs above (234 B for the issue's input). But in an @media rule that merges with an equal @media rule after it, the @media arm minifies the merged list again, and the !important rule is deleted. Main prints color:red!important there, so that is a new wrong output.

The cause of both is StyleRule::is_duplicate. It takes two rules with one selector as duplicates when one has color:red!important and the other color:blue, or when they hold different custom properties. #33684 corrects it. After #33684, the merge can come back as a size step. The @media arm has these defects on main today and does not change here.

Empty named @layer blocks

Releases: 1.3.1 prints @layer a{}@layer b{}@layer b{x{color:red}}@layer a{x{color:#00f}} for the input in Problem. 1.3.2, 1.4.2, 1.4.4 and main print @layer b{x{color:red}}@layer a{x{color:#00f}}.
Output form. The block is 1 byte longer than @layer a;, and adjacent blocks do not fold to @layer a,b;. The input in Problem is 66 B (esbuild 0.28.1: 64 B). esbuild and lightningcss print the statement, also inside a style rule, where WPT css/css-nesting/nesting-layer.html says it has no effect and Bun's parser rejects it. A statement outside style rules needs a flag in MinifyContext that each style rule with nested rules saves and restores (css: keep empty named @layer blocks as @layer statements #38480). A maintainer can ask for the statement form as a follow-up.
Not minified, a kept empty block prints one line that holds only indentation, as an empty @container block does on main.
bun build still loses a layer declaration in two linker cases that this PR does not touch, on main and in esbuild 0.28.1: the layer(x) copy of a file that is also imported without a layer, and a name that is declared only under @media. They have no tracking issue.

Chrome. The computed colour is equal for source and output in 76 of 76 probes (main: 20 of 76). The probes put an empty named block at the top level, in @media, @supports, @container, @scope, @starting-style, a style rule and a parent layer, and put same-condition @supports rules at the top level, in a style rule and in a merged @media rule. Each runs with and without --minify and --target=bun.

Cost for stylesheets that hit neither arm

CssRuleList<DefaultAtRule>::minify: 1,561 B to 1,513 B. .text +0 B. The stripped binary keeps its size (107,730,656 B).
gdb hit counts of each instruction of that function on daisyui.css (969,259 B): 1,413,077 on main, 1,410,925 here. The equal-condition test for each @supports rule is gone. The two new instructions of the layer arm ran 0 times in 2,590 layer-block visits.
149 stylesheets (113 of daisyUI 5.5.18, the 36 of this repo) in 5 modes: 0 of 745 outputs differ from main.
valgrind and perf are not installed here, so there is no whole-process instruction count.

Layer order with imports (the model and generator of #41572, 1,000 graphs, --empty-blocks)

Build	right	right on main, wrong here
main	99
this PR	261	1 whole check (seed 789), 2 winning rule (seeds 9, 504)
this PR with #41572	994	0

The three graphs have an @layer x, y; statement ahead of an @import of a file with an empty block. Main is right there only because it deletes the block. The 6 graphs that stay wrong with #41572 are wrong without empty blocks too. The row with #41572 is from an earlier commit of this branch with the same LayerBlock arm. The graphs hold no @supports rule.

@namespace. a.css = @layer a{}, entry = @import "./a.css";@namespace svg url(http://www.w3.org/2000/svg);svg|circle{fill:red}. This PR prints @layer a{}@namespace ..., and Chrome keeps 1 of 3 rules. Main prints nothing for a.css. With a.css = x{color:red}, main, this PR and esbuild all print the rule ahead of @namespace.

Self-review concerns not taken (7 of 31)

Split into two PRs, the @supports arm first (2 concerns). The layer line needs the @supports change, so they stay together. If the output form of the layer block holds up review, the layer line and its tests can move to a follow-up.
One mechanism for the @media and @supports arms: a shared helper, a deferred minify, or a port of the lightningcss pre-pass (3 concerns). Each puts rules of different source rules through one duplicate check, which loses declarations until css: compare custom property names in PropertyId equality and importance in duplicate-rule detection #33684.
Keep the second minify until the default targets move above Chrome 88 (1 concern). The second minify prints wrong output below and above that version.
Land css: compare custom property names in PropertyId equality and importance in duplicate-rule detection #33684 first (1 concern). The arm here does not depend on it.

The self-review asked for the merge with no second minify. A later code review found the merged @media case above, so this PR keeps the rules apart, which the self-review named as the other correct shape. Taken in part: the tests for long runs are in the supports block of css.test.ts, not in duplicate-declaration-merge-hang.test.ts.

Related PRs

css: compare custom property names in PropertyId equality and importance in duplicate-rule detection #33684 corrects the duplicate-rule check. It is the condition for a merge of @supports rules.
css: parse (property: value) @supports conditions as declarations and merge same-condition rules #38589 carries a merge of the @supports rules and Fixes #24770. It needs a rebase.
css: emit @layer statements that precede @import before the imported files #41572 names css: keep empty named @layer blocks as @layer statements #38480 as a prerequisite. This PR takes that place. Its EmptyLayerBlock case needs the block form.
css: minify a run of merged same-query @media rules once #38531 planned to convert the @supports arm to a deferred minify. That does not apply now.
test/bundler: reject api paths that name an input file, fix the four tests that did #38497 re-records the at-layer/007 output. The PR that lands second updates it.

…blocks The Supports arm of CssRuleList::minify dropped the second of two adjacent @supports rules with the same condition, with its content. It now minifies the second rule and appends its rules to the first. Each rule is minified one time. The LayerBlock arm dropped every @layer block whose rule list was empty after minify. An empty named block declares the place of its layer in the cascade order, so the arm now drops only an empty anonymous block.

robobun · 2026-10-24T10:32:20Z

^{Updated 8:52 AM PT - Oct 24th, 2026}

✅ @robobun, your commit ec4357801f4fecb05d66150026a383182d768789 passed in Build #132002! 🎉

🧪 To try this PR locally:

bunx bun-pr 46733

That installs a local version of the PR into your bun-46733 executable, so you can run:

bun-46733 --bun

robobun · 2026-10-24T10:32:26Z

Status: open, ready for review. CI passed for ec43578 (build 132002).

Reproduced with bun build --minify on bun 1.4.2, bun 1.4.4 and main 179664e:

@supports: the five-block input of bun bundler cannot handle css @supports #24770 prints .test{box-shadow:none;border:1px solid red}@supports (color: blue){.test{border-top:1px solid #00f}}. This branch prints all five declarations.
@layer: @layer a{}@layer b{}@layer b{x{color:red}}@layer a{x{color:blue}} prints @layer b{x{color:red}}@layer a{x{color:#00f}}. This branch keeps the two empty blocks, as bun 1.3.1 does.

Proof: the layer and supports blocks of test/js/bun/css/css.test.ts, and the css/CSSAtLayerEmptyBlock* and css/CSSAtSupportsSameCondition* cases of test/bundler/esbuild/css.test.ts.

coderabbitai · 2026-10-24T10:37:47Z

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Recent review info

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Source mode: Auto
Repository: oven-sh/bun
Branch: robobun/feb45b59/css-keep-empty-named-layer
File: .coderabbit.yaml
Commit: ec43578
Layers: Repository YAML
Review profile: ASSERTIVE
Plan: Advanced
Run ID: dbecb7cc-0570-4c60-b275-2a487c441205

Commits

Reviewing files that changed from the base of the PR and between ae850ca and ec43578.

Files selected for processing: 3

src/css/rules/mod.rs
test/bundler/esbuild/css.test.ts
test/js/bun/css/css.test.ts

Files with no reviewable changes: 1

src/css/rules/mod.rs

Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 0 remain after this review.

Walkthrough

The CSS minifier now retains non-empty adjacent @supports rules with matching conditions. It also retains empty named @layer blocks while dropping empty anonymous blocks. Bundler and runtime tests cover layer ordering, imports, nested rules, minification, and prefixing.

Changes

CSS minification

Layer / File(s)	Summary
Named layer retention `src/css/rules/mod.rs`, `test/bundler/esbuild/css.test.ts`, `test/js/bun/css/css.test.ts`	Empty named `@layer` blocks remain, while empty anonymous blocks are removed. Tests cover layer ordering, imports, nested layers, and minified output.
`@supports` rule retention `src/css/rules/mod.rs`, `test/bundler/esbuild/css.test.ts`, `test/js/bun/css/css.test.ts`	Adjacent matching-condition `@supports` rules proceed through nested-rule minification. Tests cover declarations, nested rules, output targets, prefixing, and internal rule counts.

Possibly related PRs

oven-sh/bun#41572: Changes CSS import and @layer ordering, which relates to preserving cascade-layer order.

Suggested reviewers: jarred-sumner

Priority: Normal

Severity of issue fixed: Medium

Merge Risk: Minimal · up to ec435

No actionable merge-blocking issue is established; the change is mergeable after normal checks.

Pre-merge checks: 3 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Out of Scope Changes check		Issue `#24770` covers loss of declarations from adjacent same-condition `@supports` blocks. The PR also changes `LayerBlock` minification to retain empty named `@layer` blocks and adds layer-order and i…	Remove the empty named `@layer` behavior and its tests from this pull request, or link an active issue that explicitly requires this behavior and update the pull request scope.

Passed checks: 3 passed

Check name	Status	Explanation
Description check		The description explains the problem, fix, verification, tradeoffs, and scope. It does not use the exact template headings, but it provides the required information in equivalent sections.
Title check		The title is concise and accurately identifies both primary changes: preserving same-condition `@supports` rules and retaining empty named `@layer` blocks.
Linked Issues check		Issue `#24770` requires all adjacent nested `@supports` blocks with the same condition to remain present with their declarations. The `Supports` arm no longer drops a rule only because the previous rule…

Full details: Linked Issues check

Explanation

Issue #24770 requires all adjacent nested @supports blocks with the same condition to remain present with their declarations. The Supports arm no longer drops a rule only because the previous rule has an equal condition. The bundler tests cover the five-block reproduction for default and bun targets and verify all five declarations.

Full details: Out of Scope Changes check

Explanation

Issue #24770 covers loss of declarations from adjacent same-condition @supports blocks. The PR also changes LayerBlock minification to retain empty named @layer blocks and adds layer-order and import tests. This changes separate cascade-layer behavior that #24770 does not require. The connection to nested @supports does not make the independent named-layer behavior necessary for the five-block issue reproduction.

Resolution

Remove the empty named @layer behavior and its tests from this pull request, or link an active issue that explicitly requires this behavior and update the pull request scope.

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robobun · 2026-10-24T10:39:40Z

On the out-of-scope check: the @layer change is one of the two stated goals of this PR (see the title). It is not a side change.

There is no issue to link for it. It is a regression from 1.3.1 to 1.3.2 (fix(css): process color-scheme rules inside @layer blocks #24034) that no user reported. The Problem section says so.
The two arms depend on each other in one direction. With the layer line alone, a kept @supports (c){@layer a{}} makes the unchanged Supports arm delete the @supports (c){...} rule after it. So the layer line cannot land before the merge.
If a maintainer wants the bun bundler cannot handle css @supports #24770 fix alone, the layer line and its tests can move to a follow-up with no other change.

claude

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

Additional findings (outside the current diff — GitHub can't attach inline comments there):

🟣 test/bundler/esbuild/css.test.ts — Maintainers relying on the esbuild at-layer snapshot suite get no signal for this PR's empty-layer output change: the assertion at test/bundler/esbuild/css.test.ts:2005 cannot fail. api.expectFile(join(file)) resolves through readFile(path.join(root, file)) (test/bundler/expectBundled.ts:1475), so it snapshots the input fixture, not the file in /out. The PR description says the at-layer/007 output is re-recorded by another PR, which assumes this test checks output. Fix: make the loop read the bundled file (join("/out", file)) and re-record the snapshots, or delete the vacuous loop; verify the new snapshots show the kept @ layer blocks.

Why this was flagged

Trigger: any change to CSS bundler output for the esbuild at-layer / url-format fixtures, including this PR's change to keep empty named layer blocks at src/css/rules/mod.rs:552. The test at test/bundler/esbuild/css.test.ts:2003-2006 iterates files and calls api.expectFile(join(file)).toMatchSnapshot(file). expectFile is file => expect(readFile(file)) at test/bundler/expectBundled.ts:1475 and readFile joins against the fixture root, so the snapshotted text is the source CSS. Output is written to outdir "/out" (css.test.ts:2001), which the loop never reads. Consequence: a regression in any of these fixtures passes CI, and the PR's stated plan that at-layer/007 output is re-recorded by whichever PR lands second is based on a check that does not exist. The dismissal called it pre-existing, which it is, but the PR explicitly depends on it as coverage for this change and REVIEW.md requires every assertion to be able to fail. Remedy: point the loop at /out and re-record.

Verification: pre-existing. test/bundler/esbuild/css.test.ts:2005 is api.expectFile(join(file)).toMatchSnapshot(file), so join(file) is the fixture path. readFile does readFileSync(path.join(root, file)) (expectBundled.ts:1462-1463), the source file, not /out. The base commit has the identical line; the PR's own test css/CSSAtLayerEmptyBlockKeepsLayerOrder asserts output for the same input.

The Supports arm no longer merges a rule into an equal-condition rule before it. The Media arm minifies a merged @media rule a second time, and the duplicate-rule check then compared rules that came from two @supports rules and deleted one of them. Each @supports rule now keeps its own rule list.

robobun · 2026-10-24T12:02:04Z

On the css/CSSAtImportConditionsFromExternalRepo loop (test/bundler/esbuild/css.test.ts:2005): confirmed. It reads the input fixture and not the file in /out, so it cannot see this change. It is the same on main.

test/bundler: reject api paths that name an input file, fix the four tests that did #38497 points that loop at /out and re-records the snapshots. That change stays there.
This PR does not depend on the loop. css/CSSAtLayerEmptyBlockKeepsLayerOrder asserts the output for the same at-layer/007 input, minified and not.

The other review threads are answered inline. ec43578 changes the @supports arm: it keeps each rule and no longer merges them, which removes the !important case in a merged @media rule.

claude

Code review found no issues

No high-confidence issues detected in this change.

@mikamikasuki

### What does this PR do? Consolidates the open pull requests on the CSS parser (`src/css`: parsing, minifying, lowering, printing, CSS modules) into one. 179 open pull requests were in scope. Each was judged by one rule: **the output must mean to a browser what the input meant.** Every claim was run through a build of `main`, a build with the change, lightningcss 1.33 and Chromium 156 (CSSOM, computed values, painted pixels for colors and gradients), and counted against 10,116 real stylesheets: the published CSS of about 190 popular npm packages, plus stylesheets from open source repositories. 107 land here, whole or in part, most of them cut down. The source diff is net negative. Of the 10,116, 9,509 are stylesheets that someone ships. The rest are test fixtures of other tools (521), Tailwind v4 source that needs its plugin (59) and SCSS, PostCSS or template source (27). | 9,509 shipped stylesheets | `main` | this branch | |---|---|---| | fail to build | 71 | 35 | | ... of those, valid CSS that Bun rejects | | 0 | | minified output reads differently in Chromium than the input | 552 | 2 | | CSS modules with a name that is referenced but not defined, or exported but not defined | 166 | 0 | | all 10,116 | `main` | this branch | |---|---|---| | fail to build | 192 | 141 | | minified output reads differently in Chromium than the input | 593 | 15 | | build on `main`, fail here | | 0 | | right on `main`, wrong here (minified, and at the default targets) | | 0 | | size of the minified output, no targets | 162.32 MB | 162.47 MB (+0.093%) | | size of the output at the default targets | 224.1 MB | 210.2 MB (-6.2%) | A syntax error is a build error, with one exception: the IE star hack `*zoom: 1` is skipped, because tachyons and handsontable ship it. The 35 shipped stylesheets that fail are invalid CSS: 28 have a selector that Chromium drops (semantic-ui-css, swiper, @ionic/core, Adobe Spectrum, xp.css), 2 an `@import` after other rules, and one each `@supports not X and Y`, a nested `@keyframes`, a stray comma, a selector list in `::slotted()`, `@media screen\0` (govuk-frontend). The 2 that read differently fold a `color-mix()` inside a gradient (see the known gaps). The growth of the minified output is rules and declarations that `main` deleted by mistake: 87 KB of rules, 45 KB of prefixed declarations that only browser targets can rule out. #### Popular CSS that `main` breaks today - normalize.css, modern-normalize, sanitize.css: `font-family: monospace, monospace` is collapsed to `monospace`, which changes the font size from 16px to 13px (#46662). - CKEditor 5, Tabler, Carbon, VitePress, Ionic, PatternFly, Excalidraw: a rule of custom properties is deleted as a "duplicate" of a later rule with the same selector and other custom properties. 37 stylesheets lose 1,681 custom properties (#33684). - Material Components, daisyUI, DocSearch, plyr: a `var()` size loses to its own fallback (#42101). - nextra, daisyUI, CKEditor: declarations written after a nested rule are moved above it (#45462). - antd 4, purecss, govuk-frontend: browser hacks in selector lists start to apply everywhere (#42100). - foundation-sites, flatpickr, 98.css, blueprintjs, scalar, react-datepicker: the build fails on valid CSS (#46756, #38552, #38534). tachyons, handsontable: it fails on the IE star hack (#46751). - nodejs.org, Mantine: a CSS module exports classes that no rule defines, because the `@apply` or `@mixin` in them is moved out of the rule (#46759). - hint.css and 42 more: `z-index: 1000001` and `1000000` become the same number (#46715). - Adobe Spectrum: `--version: 5.0.1` prints as `5.1`. - A quarter of all stylesheets: minified output has a stray space in `align-items: center` (#41993). #### Behavior changes to look at first 1. **`light-dark()` is kept as written at the default browser targets** (#42009). The lowered form `var(--buncss-light, a) var(--buncss-dark, b)` is invalid unless a compiled `color-scheme` covers the element: 184 of the 215 real stylesheets that get it never define the variable, so every browser drops the declaration. The docs section on the polyfill is removed. 2. **`border` is not built from longhands** (#42072): `border-width:0;border-style:solid;border-color:red` stays three declarations, because `border` also resets `border-image`. 305 real stylesheets grow by 30 KB in total; `bootstrap.min.css` by 113 bytes, 5 gzipped. No real stylesheet was found where the old merge shows, so this is the one to drop if size matters more. 14 existing expectations change. 3. **A CSS module no longer exports names that only occur inside `:global(...)`** (#45487). 73 real modules lose 202 export keys, all of them names that no rule has. `import { g }` or `composes: g` of such a name becomes a build error. 4. **Container names and `@counter-style` names stay global in a CSS module** (#45247 by @mikamikasuki). `main` hashes the `@container` name but not `container-name`, so a named query never matches. postcss-modules (Vite, css-loader, Next.js) keeps them global; lightningcss and esbuild hash both. Global keeps a container that is queried from another file or from JS working. 5. **Syntax errors are build errors, also two that `main` lets through**: a media query that stops short (`@media screen and {`, `@media not {`, `@import "a.css" screen and;`) and `@media (width <) foo {`. No stylesheet of the 10,116 has either. A condition that is valid by the grammar though no browser knows it, `( ... )` or `fn( ... )`, is kept as written (#46756): `(min-width:0\0)`, `(not(hover))`. 6. **Declarations after a nested rule stay where they were written**, as a nested declarations rule (#45462). Changes the output of 199 real stylesheets; in a fuzz of 2,850 nested programs compared by computed style, `main` was wrong in 1,056 and the pull request's code, before it was trimmed here, in none. 7. **`Bun.color`**: `color()` inputs are converted instead of returned as a string (`Bun.color("color(srgb 1 0 0)", "hex")` was `"color(srgb 1 0 0)"`); `currentColor`, system colors and `light-dark()` give `null` in the converting formats instead of the keyword as a string; a relative color or `color-mix()` outside sRGB is clipped, not gamut-mapped (`hsl(from oklch(100% .399 336.3) h s l)` is `#ff5bff`, was `#fff`). 8. The `:-moz-any()` and `:-webkit-any()` copies of rules with `:is()` are gone at the default targets (#45945, #42029). The `:-webkit-any()` copy outranked the original rule: `.title{color:blue} :is(h1,h2,h3){color:red}` painted red. 5.8% of all default-target output was the `:-moz-any()` copies. 9. `Property` grows from 216 to 224 bytes: `Url` stores its text as a slice so that two `url()` values can be compared (#46895). The pull request packed a pointer and a `u32` behind `unsafe` to avoid that. #### Not in this PR, needs a decision - **Raise the default browser targets (#40369) and add a `cssTarget` option (#40368).** Today: Chrome 80, Edge 80, Firefox 78, Safari 14, Opera 67, and no public way to change them. At Vite 8's floor the output of the 9,924 stylesheets is 15.3% smaller (3.9% gzipped). About a dozen fixes here only matter at today's floor, and code for older targets is unreachable without the option. - Vendor prefixes for `user-select`, `appearance`, `hyphens` and five more (#45814): 1,116 real stylesheets lack a prefix they need at the default targets. And for `backdrop-filter` (#44648): 173. - Clip instead of gamut-map in the sRGB fallback of a plain wide-gamut color: deletes about 90 lines, changes every pinned fallback hex. If the mapping stays, #44647 by @oddharsh makes it closer to what browsers paint (colors off by 2/255 or more: 168 to 110 of 3,170 real ones). #### Changes **Rules: duplicates, merging, nesting, fallbacks** - The `:-moz-any()` copy of every rule with `:is()` or `:not(a, b)` is gone: Firefox 78 has `:is()` (#45945). 1,164 real stylesheets - Keep declarations written after a nested rule in place with a nested declarations rule (#45462). 379 real stylesheets - With nesting compiled away, an unknown at-rule inside a style rule was moved out of it: `.wrapper{@apply flex}` printed a bare `@apply flex;` and the rule was gone, so a CSS module exported a class that no rule defines. It now stays in a block under the compiled selector. 72 real modules with Tailwind's `@apply`, 25 with Mantine's `@mixin` (#46759). 316 real stylesheets - Keep !important in the rules generated for logical properties and color-scheme (#38597). 311 real stylesheets - With browser targets, remove an earlier duplicate rule only when its declaration block is equal (#46861). 171 real stylesheets - Compare custom property names in `PropertyId` equality and importance in duplicate-rule detection (#33684). 40 real stylesheets - Keep same-condition @supports rules and empty named @layer blocks (#46733). 31 real stylesheets - Keep target-added vendor prefix passes on cloned and re-minified rules, and never print a bare `:full-screen` (#43339). 12 real stylesheets - Keep the selectors on rules added for color fallbacks and logical properties (#43240). 3 real stylesheets - A later declaration of a property drops the `@supports` color copies of the earlier one, which printed after the rule and won (#45560). 2 real stylesheets - Do not merge style rules in a stylesheet that holds `revert-rule` (#46953). 1 real stylesheets - `@import ... supports(-webkit-mask: none)` kept its prefix (#38589) **Properties and shorthands** - Print align-items:center without a stray space when no safe/unsafe keyword is given (#41993). 3,317 real stylesheets - Emit the physical longhands instead of the inset shorthand for targets without it (#38583). 1,452 real stylesheets - Keep the stops of an upward gradient as written, and drop a `50%` hint only at the midpoint (#46790). 104 real stylesheets - Emit unparsed border-block-* declarations when compiling logical borders (#38582). 89 real stylesheets - Keep repeated font families and dedupe only the inserted system-ui fallbacks (#46662). 88 real stylesheets - A `width` / `height` / `min-*` / `max-*` with `var()` or `env()` was printed before the plain fallback written above it, so the fallback won (#42101). 26 real stylesheets - Keep box-orient and box-direction when flex-direction or flex-flow follows (#45672). 16 real stylesheets - Write held prefixed background declarations at every flush (#46882). 6 real stylesheets - Derive equality for Size2D and BorderRadius, and keep every prefix in the border-radius handler (#45665). 2 real stylesheets - Print `none` instead of an empty border-image, mask-border or mask-box-image value (#45143). 2 real stylesheets - Keep font/transition sub-properties on the side of the shorthand they were written on (#41990). 2 real stylesheets - CSS-wide keywords are not names: `font:12px Arial;font-family:inherit` printed `font:12px inherit`, `color-scheme:inherit` and `color-scheme:auto` printed `normal`, `transition-property:unset` was folded into `transition` (#42022). 2 real stylesheets - Only fold font longhands into `font` when the rule declared a `font` (#45459) - Keep a font family name quoted when its identifiers do not read back as the same name (#46606) - Stop rewriting valid positions into ones browsers reject (#45197) - `border` is no longer built from longhands unless the rule also sets what `border` resets (`border-image`), and an author's `border-image` reset is no longer dropped. `transition` is likewise only written where the rule wrote one, because it resets `transition-behavior` (#42072) - `border; border-image: <gradient>; border-image-slice: 1` lost the gradient in every default build (#42018) - A prefixed `linear-gradient` keeps its direction, written or generated: the prefixed syntax names the starting side and measures angles differently (#46697) **At-rules** - Print @font-face format() keywords as strings (#37979). 439 real stylesheets - Print @supports `not` conditions without a leading space (#38559). 88 real stylesheets - Reset after the number token of a unicode-range (#45835). 33 real stylesheets - Keep `<general-enclosed>` media and container conditions as written (#46756). 24 real stylesheets - A strict range such as `(width < 768px)` lowered for old targets still matched at exactly 768px. The step is now `.02px`. The `not all and` form for other units is not taken (#46806). 18 real stylesheets - Keep scanning an @supports condition after a nested block (#38552). 9 real stylesheets - Every `@counter-style name` printed as `@counter-stylename`, which browsers drop (also #46692). In a CSS module the name now stays global, like the `list-style` that refers to it (#33679). 9 real stylesheets - Keep a keyframe whose block has an invalid item (#46288). 2 real stylesheets - Read an @Property initial-value on the stylesheet's parser, keep an unmatched value as written (#45806). 2 real stylesheets - `@media {}` is `all`. A media query that stops after `and`, `or` or `not` (`@media screen and {`) is a build error; `main` printed `not all` or dropped the condition, silently. The `@layer` and type selector parts are not taken (#46869). 2 real stylesheets - Accept a name-only `@container` and a `style()` feature with no value (#46689). 1 real stylesheets - Parse `<timeline-range-name> <percentage>` keyframe selectors (#45652) - Allow `or` inside parentheses after a media type (#45088) - Print the `=` comparison in media and container queries as `=` (#45071) - Parse the @font-palette-values font-family descriptor as a list of family names (#45484) **Selectors** - Never wrap a selector list in :is() (#46787). 414 real stylesheets - Stop emitting :-webkit-any() copies of :is() that outrank the original rule (#42029). 373 real stylesheets - With browser targets, a selector list with a vendor-prefixed member was split, which revives hacks that rely on other engines dropping the whole list: `.s *::-ms-backdrop,.s{flex:auto}` applied everywhere. Rewritten; the list is printed as written (#42100). 42 real stylesheets - Print the vendor prefix on downleveled :any-link, :read-only, :read-write, :placeholder-shown and :autofill (#42131). 12 real stylesheets - Fix nesting lowering of a `&` that does not lead its selector (#43047). 8 real stylesheets - `@scope`: `&` and implicit selectors compile to `:where(:scope)`, relative selectors and bare declarations parse (#43165). 7 real stylesheets - Only reject bare :global / :local pseudo-classes in CSS modules (#38534). 5 real stylesheets - Stop truncating `@scope to (...)` and stop replacing `&` in the scope-end selector (#43146). 2 real stylesheets - `[xl|href$=".pdf"]` lost its namespace prefix (#45159). 2 real stylesheets - Match An+B keywords exactly and take only `+` as the leading sign (#42953). 1 real stylesheets - Parse ::details-content and ::picker() like ::part() (#45504). 1 real stylesheets - Parse :window-inactive as an ordinary pseudo-class, and accept it after ::selection (#45815) - Print :nth-col(1) and :nth-last-col(1) in functional form (#41575) - `li::before::marker` failed the build (#45810) - `&` inside `:nth-child(... of S)` is resolved when nesting is compiled away. The second selector serializer (326 lines) is deleted (#43024) **Colors and `Bun.color`** - Keep light-dark() as written under the default browser targets (#42009). 227 real stylesheets - Number and percentage channels parse in `lab()`, `lch()`, `oklab()`, `oklch()`, `hsl()` and `hwb()`; the hue of `lch` / `oklch` is no longer scaled by alpha (#45084). 130 real stylesheets - `Bun.color` returns `null` for `currentColor`, system colors and `light-dark()` in the converting formats, instead of the CSS keyword as a string (#33047). 130 real stylesheets - 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 (#45362). 31 real stylesheets - Keep rotate: 0deg out of none, and keep the origin alpha in relative colors (#38486). 13 real stylesheets - Compute the D50 reference white in f32 so greys convert to lab(l 0 0) like lightningcss (#38535). 7 real stylesheets - Return a missing hue for a grey from conversions into lch, oklch, hsl and hwb (#45201). 5 real stylesheets - Fix color(a98-rgb), srgb-linear relative colors and color-mix(in xyz-d50) (#38487). 4 real stylesheets - Match named colors without regard to ASCII case (#45092). 3 real stylesheets - Fold min(), max() and clamp() over plain numbers (#39515) - `calc(l - .08)` in a percentage channel of a relative color became NaN and printed `#000` (also #45970) (#45971) - Bun.color: convert color() inputs instead of returning them as a CSS string (#38488) - A `color-mix(in srgb, ...)` result outside sRGB prints `color(srgb ...)` instead of a gamut-mapped hex (#39269) **`calc()` and numbers** - Rounding to 6 significant digits ate into the integer part: `z-index:2147483647` became `2147480000`, `max-width:9999999px` became `10000000px`, `--x:1234567.5` became `1234570`. It now only cuts a fraction short: a value kept as tokens prints its source text, a typed value prints every integer digit and at least one digit of the fraction (#46715). 43 real stylesheets - A number without a unit is no longer repaired into `px` (`height:1.5715` is dropped by a standards-mode browser, so it prints as written), `calc(-1 * 0)` is not a length, and on a property of the range [0,∞] (sizes, border radii, padding, border and outline widths, gap, `flex-basis`, `font-size`) a `calc()` that folds below zero prints `0` instead of an invalid negative literal, and a negative literal no longer replaces the valid declaration before it (`width:10px;width:-5px` printed `width:-5px`) (#42104). 24 real stylesheets - Print numbers from the f32's shortest digits, not its f64 expansion (#38565). 17 real stylesheets - Print negative numbers below 1 with a single minus sign (#38516) - Keep math functions over two different length units as written (#38489) - Fold rem() with a truncated quotient, so it keeps the sign of the dividend (#32286) - A one-argument `min()` / `max()` around a sum printed without any function (`width:100% - 20px`) (#46145) - `round()` breaks ties upward and handles a negative step as browsers do (#45249) **CSS modules** - A CSS module exports its `@keyframes` names (`styles.spin`). Hashing is unchanged (#46720). 145 real stylesheets - A name inside `:global(...)` prints plain but was exported as `name_<hash>`, a class no rule has (also #38573) (#45487). 90 real stylesheets - Keep container names global in CSS modules (#45247). 6 real stylesheets - Tag the keyframes name when an `animation` value falls back to raw tokens (#46034). 6 real stylesheets - In a plain stylesheet `composes: a from "./b.css"` was treated as an import: the file was inlined and an absolute path printed (#45961). 4 real stylesheets - Drop a rejected composes instead of failing the file at print time (#38538). 1 real stylesheets - Keep every rule's custom properties for the composes conflict check (#38523) - Keep composes working after a nested rule in the same block (#38520) - `IdentOrRef::hash` read 2 of 16 bytes, so in release builds every class of a module hashed alike (#30784) **Tokens, `url()`, diagnostics** - The block skip stack no longer allocates on every block exit (#39533) - Report the diagnostic's line without its line breaks in position.lineText (#38342). 192 real stylesheets - Compare `url()` values by their text (#46895). 26 real stylesheets - Values kept as tokens keep the blanks they were written with instead of having them re-invented at print time: `attr(x type(<number>))` printed `type(< number> )`, which browsers drop. Minified output still drops the blanks that cannot matter (#45577). 19 real stylesheets - The IE star hack (`*zoom:1`) failed the whole build. The declaration is skipped, as every browser does. It is the one syntax error that builds: tachyons, handsontable and jasmine ship it (#46751). 8 real stylesheets - The path of a copied asset spliced into `url()` is always quoted, so a file name with a space or a parenthesis cannot end the `url()` (#41557). 7 real stylesheets - Move the skip loop of parse_until_before out of the generic body (#39520) - Remove dead code from bun_css: the unused `impl Default for Property` (#45618) - Default the at-rule and qualified-rule parser traits, drop the url_to_css shim (#31999) **Speed** Four inputs that took quadratic or exponential time, or produced output out of proportion, are now linear, with no new limit and no extra pass (#31639, #42243, #38531, #43421). **Tests only** - `css-fuzz.test.ts` never ran in CI and timed out on debug builds; it is a deterministic table now (#38500) - `nth-anplusb-ident.test.ts` counts heaps in a child process (#46331) - `color.test.ts` checks the ansi-16 sweep with one `expect()` (#42824) - Not CSS, but it failed this PR's CI and fails on `main` too: `ws-proxy.test.ts` on Windows, "a message sent during a TLS renegotiation arrives after it". The helper server now renegotiates from the callback of the frame's write (#46170) **No pull request covered these** - `--v: 5.0.1` printed as `5.1`, `--w: 1.0.0` as `10`, `--x: 1+2` as `12`, `2n+1` as `2n1`: a number token that touches another keeps its source text. - `color-scheme: auto` printed as `normal`, and `color-scheme: dark light` as `light dark`. - A bare `:global` or `:local` in a CSS module sets the scope of the names after it, up to the end of the selector, as in css-loader. It was a build error. - The scrollbar states (`a:horizontal`, `::-webkit-scrollbar{&:horizontal{}}`) are valid in a plain compound, as in WebKit and Blink. - Without browser targets, `-webkit-box-align`, `-ms-flex-align` and the like were dropped next to their standard form (flatpickr, foundation-sites, materialize-css: 28 stylesheets), and `-webkit-fill-available` before `-moz-available` (beercss). - `(-o-min-device-pixel-ratio: 2/1)` printed as `2`. - `sin()`, `cos()` and `tan()` are exact at multiples of 90deg: `calc(100px * cos(90deg))` printed `-.00000437114px`. - A `display-p3` color at targets that have P3 but not the other `color()` spaces (Safari 10.1 to 14) got itself as a fallback. - `width: rem(-18px, 5px)` folds to a negative literal once `rem()` is right; it prints `0`. - Consecutive nested declarations rules, which arise when same-query `@media` blocks in one rule merge, are folded into one and count against the existing selector expansion limit as the `&{}` rules they replace did. #### Known and not fixed here - Four longhands are still folded into a `transition` the author did not write where the targets add a prefixed copy of a property (`mask-image` at Chrome 80): only the shorthand gives the copy its own times. - A later `background`, `box-shadow` or `border-*` declaration still loses to the `@supports` copy of an earlier `var()` value of the same property with a wide-gamut color. Same on `main`; #45560 is taken for the properties of the fallback handler and for unparsed values only. - A merged rule prints the unprefixed name in the passes of the rule it absorbed: `.d::-webkit-input-placeholder{}.d::-moz-placeholder{}` prints `::placeholder` for the second (#42106, not taken: it breaks the merge of prefix-equivalent rules). - The `border`, `outline` and `font` shorthands, `line-height`, `scroll-padding`, `background-size` and a few more still print a `calc()` that folds below zero as a negative literal. No real stylesheet has one. - `all` is not a property Bun knows, so `a{margin-top:1px;all:unset}` prints `all` first. Same on `main`. - A selector kept as raw tokens loses the blank before `*` when minified: `--sel: .dark *` prints `.dark*`. Same on `main`. - `width:calc(0% + 12px)` folds to `12px`, which lays out differently in a table cell. Same on `main`. - The `:dir()` / `:lang()` rules of a lowered logical property print before the nested rules of their rule, so a logical declaration written after a nested rule of equal specificity still loses to it. Same on `main`. - A fraction is still rounded to 6 significant digits (`12345.678` prints `12345.7`, `33.3333333333%` prints `33.3333%`). Keeping every digit would add up to 282 KB over the corpus. - `@keyframes "quoted"` is hashed in a module but not exported. - In a module, a keyframes name inside a `var()` fallback is not hashed: `animation: var(--enter, fade 1s)`. Same on `main`. - A `color-mix()` or relative color folded to a hex inside a gradient changes the interpolation from Oklab to sRGB (VS Code, clawhub). Printing `color(srgb ..)` instead needs a fallback for `mask-image`, `list-style-image` and custom properties at the default targets, which have none. - A comma list in `@container` still fails the build. - After a bare `:global`, names that are hashed at print time (`::view-transition-old(foo)`) are still hashed; inside `:global(...)` they are not. #### Pull requests in scope that do not land here - Duplicates of a pull request that lands here: #36961 (of #37979), #38478 (of #46662), #38480 (of #46733), #38482 (of #46715), #38491 (of #38500), #38508 (of #45201), #38513 (of #45362), #38573 (of #45487), #38640 (of #38500), #45525 (of #45247), #45589 (of #46756), #45970 (of #45971) - Already fixed on `main`: #38674 - Right, but for input nobody writes (0 of 10,116 stylesheets, and not new syntax): #31916, #38526, #38592, #39349, #41139, #41801, #42042, #42484, #43078, #45604, #45908, #46086, #46221, #46224, #46273, #46694, #46707, #46737, #46755, #46816, #46870, #46934 - Right or harmless, but not worth their code, their bigger output or the behavior they change: #30905, #31924, #33332, #34426, #36225, #38006, #38512, #38547, #38551, #38561, #38572, #38576, #38607, #38623, #38667, #38692, #42106, #42912, #42918, #43407, #43434, #44700, #45442, #45546, #45747, #45759, #45825, #45946, #46061, #46127, #46206 - Need a decision (see above), stay open: #40368, #40369, #44647, #44648, #45814 - Stays open: #45787 Out of scope: CSS linking and chunking in the bundler, the dev server, docs-only pull requests, and repository-wide sweeps that touch `src/css` in passing. Thanks to @mikamikasuki (#45247), @alii (#39520, #31999) and @oddharsh, whose measurements on #40369 and #41990 were used. Fixes #16727. Fixes #24770. Fixes #30772. Fixes #44704. Fixes #45089. Fixes #45237. Fixes #46872. ### How did you verify your code works? - The 35 CSS test files pass on a debug build, also with `FORCE_COLOR=1`. 600+ of the tests in this branch fail on a build of `main`. An earlier head of this branch passed all 179 CI jobs that ran. - The reproduction of each issue listed above was run on `main` (wrong) and on this branch (right). - Differential run over the 10,116 stylesheets, `main` against this branch, minified without targets and through `Bun.build` at the default targets: no stylesheet that builds on `main` fails; Chromium 156 compares the output with the input rule by rule (CSSOM, then computed values on a sized element in a standards-mode document, colors as painted pixels in sRGB and P3). The comparison has 195 control pairs that must read equal or different. For stylesheets that read differently on both builds, mostly because of lowering, the sets of differences were compared; the three new ones are two minifier fixtures that are wrong on both and one comparison artifact. - CSS modules are checked for consistency instead, since hashed names defeat the comparison: every reference points at a name the output defines, every export exists. - `Bun.color`: 59 inputs in 13 formats on both builds; every changed cell was read.

Jarred-Sumner · 2026-10-27T19:55:56Z

Landed in #47025 (7942008).

Keep same-condition @supports rules and empty named @layer blocks. 31 real stylesheets

robobun · 2026-10-27T20:56:18Z

Confirmed on main 7942008 with a debug build.

Both arms of CssRuleList::minify match this PR.
@layer a{}@layer b{}@layer b{x{color:red}}@layer a{x{color:blue}} keeps its two empty blocks, minified and not. The five blocks of bun bundler cannot handle css @supports #24770 all print.
The @supports rules with one condition, and empty @layer blocks cases of test/js/bun/css/css.test.ts and the css/CSSAtLayerEmptyBlockKeepsLayerOrder and css/CSSAtSupportsSameConditionKeepsAllRules bundler cases pass.

One item from the Notes is still open: #41572. Its EmptyLayerBlock case expects the statement form and now needs the block form.

robobun assigned Jarred-Sumner Oct 24, 2026

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Comment thread src/css/rules/mod.rs Resolved

Comment thread src/css/rules/mod.rs Resolved

Comment thread test/js/bun/css/css.test.ts Resolved

Comment thread src/css/rules/mod.rs Resolved

Comment thread src/css/rules/mod.rs Resolved

Comment thread src/css/rules/mod.rs Resolved

robobun changed the title ~~css: merge same-condition @supports rules and keep empty named @layer blocks~~ css: keep same-condition @supports rules and empty named @layer blocks Oct 24, 2026

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Jarred-Sumner mentioned this pull request Oct 27, 2026

css: consolidate the open CSS parser fixes #47025

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coderabbitai Bot mentioned this pull request Nov 2, 2026

css: print @layer statements before @import and keep the layers of duplicate imports #47797

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css: keep same-condition @supports rules and empty named @layer blocks - #46733

css: keep same-condition @supports rules and empty named @layer blocks#46733
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