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`revert-rule` gives a property the value it has without the style rule
that holds the keyword. The same-selector rule merge and the
duplicate-rule removal moved or deleted declarations across that rule
boundary, so `.a{color:red}.a{color:revert-rule}` printed
`.a{color:revert-rule}`. bun 1.2.11 to 1.3.14 printed both rules.
The token-list parser now records, per stylesheet, that a value holds
the ident `revert-rule`. In such a stylesheet `merge_style_rules` does
not merge, and the duplicate-rule removal does not remove the earlier
rule. Every other stylesheet is minified as before.
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Updated 10:10 PM PT - Oct 25th, 2026
✅ @robobun, your commit 4c9224964366de19b5c2d53a0235cd899d8b4af9 passed in 🧪 To try this PR locally: bunx bun-pr 46953That installs a local version of the PR into your bun-46953 --bun |
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Status How it was reproduced:
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This was referenced Oct 26, 2026
Closed
![claude[bot]](https://avatars.githubusercontent.com/in/1236702?s=60&v=4){kind=small}
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline findings, I also checked whether a revert-rule in one bundled file could fail to protect rules in another file: minify runs per source in src/bundler/ParseTask.rs:1330 and the linker only concatenates the already-minified sheets, so rules from different files are never merged with each other and the per-stylesheet flag is sufficient there. StylesheetExtra has a single construction site (src/css/css_parser.rs:2671), so the new field is always populated.
Extended reasoning...
The source diff is 22 lines across src/css/css_parser.rs, src/css/properties/custom.rs and src/css/rules/mod.rs adding a per-stylesheet may_revert_rule flag set from the TokenList ident path and read at the two style-rule merge/dedup sites, plus a large block of new CSS minifier tests. It touches no security-sensitive surface. Six confirmed inline findings (unbounded growth on re-bundle because the vendor-prefix fold is also disabled, the uncovered name-valued property path, nested-rule hoisting and logical-property expansion, and two pre-existing merge bugs) mean a human should weigh the scope of the guard; the one additional candidate, cross-file leakage of the flag, was ruled out because the bundler minifies each source separately.
Additional findings (outside the current diff — GitHub can't attach inline comments there):
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src/css/rules/mod.rs— Pre-existing: users bundling CSS with two adjacent@ supportsblocks that share a condition lose the second block's rules entirely, and this PR's guard does not reach it. At src/css/rules/mod.rs:529-534 the duplicate-condition arm doesbreak 'armwithout pushing or mergingsupp, so the rule is discarded whenself.v = rulesruns at mod.rs:642. Fix: fold the second block's body intolast_rule.rules.v(as the@ mediaarm at mod.rs:516-523 does) and setpending_at_rule_minify, so nested style rules flow through the gatedminify_style_armpath. Minify runs unconditionally in ParseTask.rs:1330, so this affects everybun build, with or without--minify. [also at: src/css/rules/mod.rs:533 - Pre-existing, left open by this fix: a stylesheet whoserevert-rulerule sits in a second@ supportsblock with the same condition loses that whole block, so the browser never sees the keyword.]Why this was flagged
Input
@ supports (display:grid){.a{color:red}}@ supports (display:grid){.b{color:blue}}throughbun build(src/bundler/ParseTask.rs:1330 callsminifyfor every CSS file,--minifyor not). InCssRuleList::minifythe second@ supportsreaches the arm at src/css/rules/mod.rs:528;last_rule.condition.eql(&supp.condition)is true, and the branch at mod.rs:529-534 executesbreak 'armwithout appendingsupp.rules.vtolast_rule.rules.vand without pushingsuppintorules.moved_rulestays false, the rule is left inself.v, andself.v = rulesat mod.rs:642 drops it. The base branch does the same, so this is pre-existing, but it is a merge site in the same minifier this PR audits: with@ supports (top:0){.a{color:red}}@ supports (top:0){.a{color:revert-rule}}therevert-ruledeclaration is still lost after merging, and the newmay_revert_ruleflag at mod.rs:868 and mod.rs:1168 is never consulted because the block never reachesminify_style_arm. The@ mediaarm at mod.rs:516-523 shows the intended shape: append the bodies and setpending_at_rule_minify.Verification: Pre-existing. Triggering condition: any stylesheet with two adjacent
@ supportsblocks whose conditions compare equal, through anybun buildof CSS. Mechanism at src/css/rules/mod.rs:528-534:break 'armskips therules.pushat line 614, and nothing appendssupp.rules.vtolast_rule.rules.v;self.v = rules;at line 642 then replaces the list, so the second block's rules are gone from the output. -
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src/css/rules/mod.rs— Pre-existing, in the merge this PR gates: users ofbun build --minifyget.a,.b{all:initial}for.a{all:initial}.b{all:unset}, so.bsilently changes meaning.merge_style_rulesdecides a selector merge fromsrc.declarations.eql(&dst.declarations)at src/css/rules/mod.rs:1201, andProperty::eqltreats any twoalldeclarations as equal regardless of keyword (src/css/properties/properties_generated.rs:5014(Property::All(_), Property::All(_)) => true). Fix: compare theCSSWideKeywordpayload in that arm (a == b), soall:initial,all:inherit,all:unset,all:revertandall:revert-layeronly merge with the same keyword; the dedup path at mod.rs:860-873 compares property ids only and is unaffected.Why this was flagged
Input
.a{all:initial}.b{all:unset}through any minify caller (src/bundler/ParseTask.rs:1330, src/bundler/transpiler.rs:3101, src/css_jsc/css_internals.rs:144) on a stylesheet withoutrevert-rule, somay_revert_ruleis false and merging runs.PropertyId::Allparses the keyword intoProperty::All(CSSWideKeyword)(properties_generated.rs:3877). Inmerge_style_rulesthe selectors differ, so it reaches src/css/rules/mod.rs:1201 and callsDeclarationBlock::eql(src/css/declaration.rs:193), which compares each pair withProperty::eql; the arm at src/css/properties/properties_generated.rs:5014 returns true for any twoAllvalues without reading the keyword. Both rules are then treated as having identical declarations and.bis appended to.a's selector list (mod.rs:1222-1240), printing.a,.b{all:initial};.b'sunsetis lost. The same happens on the base branch; this PR touchesmerge_style_rulesand leaves the comparison as is. lightningcss derives PartialEq onProperty::All(CSSWideKeyword)and does not merge these.Verification: Pre-existing: the base produces the same wrong output; the PR's
may_revert_ruleearly-return leaves the comparison untouched. Trigger:.a{all:initial}.b{all:unset}minified withoutrevert-rule. src/css/properties/properties_generated.rs:5014(Property::All(_), Property::All(_)) => trueignores the keyword, so src/css/rules/mod.rs:1201 merges the rules, discarding.b'sall:unset.
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Thanks for the review. The diff does not change. The four inline threads have replies. On the two findings outside the diff:
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Jarred-Sumner
added a commit
that referenced
this pull request
Oct 27, 2026
### 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.
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Confirmed on a debug build of main 7942008.
Two leftovers from the notes of this PR are still on main, and no open PR has them:
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Stacked on #38531. Land it first.
Problem
bun buildmerges style rules that have the same selectors..a{color:red}.a{color:revert-rule}prints.a{color:revert-rule}, and the element loses its red color. 1.2.11 to 1.3.14 print both rules.merge_style_rules(src/css/rules/mod.rs:1158) puts both declaration lists in one rule. The duplicate-rule removal (mod.rs:857) deletes the earlier rule.Fix
revert-rulein a value.merge_style_rulesdoes not merge, and the duplicate-rule removal does not remove. Other stylesheets are unchanged.test/js/bun/css/css.test.ts(40 new tests fail on 1.4.3),test/js/bun/css/,test/bundler/css/. WPT in Chrome 153: 45 of 46 (1.4.3: 26).test.failingrows, a tracking issue (Notes).Background
revert-rulegives a property the value it has without the rule that holds the keyword. Chrome 148, Firefox 150 and Safari 27 ship it..arules merged until 1.2.10 and merge again since 1.4.0 (Rewrite Bun in Rust #30412). No issue reports it.Downsides
revert-rulealone in nine properties that parse a name (css: keep css-wide keywords and percentage stretch out of the font and transition shorthands #42022). Also a keyword from a custom function orattr().Notes
The stack. The diff of this PR is 22 lines in
src/cssand the tests. Its base branch,robobun/2afc5df7/css-revert-rule-stack-base, is main449877f3e0with #38531 merged in. #38531 conflicts with main since #42948. The merge threads thestyle_rulesparameter of #42948 throughflush_pending_merges,end_merge_runandcascade_merge_with_previous. Nothing else changes.Why the stack: a flagged stylesheet does not merge the rules of a joined
@medialist. On main each join sends the whole list through the minifier again. 2,000 same-query@mediarules then cost 4.9 G instructions, where main needs 23 M. 256 of them in a nested list fail to build. #38531 minifies the joined list once, for every stylesheet. With it, every flood below costs 2.00x per doubling. The base against main: 0 of 12,362 npm outputs differ with browser targets, 2 with none (material-components-web, +180 B each).Who reported it. No bun issue. A read of the minifier found it. One report exists upstream: a comment on parcel-bundler/lightningcss#580 from 2026-01-29 ("the current property merging behavior is no longer safe"). bun 1.4.3 prints that comment's merged output. webpack 5.112 has a guard for the
revertkeywords in its CSS minifier (_ROLLBACK_REinlib/css/syntax-parser.js). Earlier CSS fixes that merged here with no issue: #33147, #42948. If the guard must wait for a bun report, the tests can stay astest.todo.Outputs (
bun build a.css --minify, release binaries).a{color:red}.a{color:revert-rule}.a{color:revert-rule}.a{margin:1px}.a{margin-top:revert-rule}.a{margin:1px;margin-top:revert-rule}.a{color:red}.b{top:0}.a{color:revert-rule}.b{top:0}.a{color:revert-rule}.a{color:revert-rule!important}.a{color:red}.a{color:red;color:revert-rule!important}div{color:red}div{color:revert-rule})..test{font-size:36px;color:green}.test{color:var(--foo, revert-rule)}, this PR prints.test{color:green;font-size:36px}.test{color:var(--foo,revert-rule)}. The comment asks for one rule without the keyword declaration. That is wrong when--foohas a value.What flags a stylesheet.
TokenList::parse_into_impl(src/css/properties/custom.rs:552) parses every value that has no typed form. The identrevert-rulethere sets the flag. It can be the whole value, avar(),env()orattr()fallback, anif()branch, or a part of a custom property. A plainvar(--x)does not set it: no stylesheet construct lets a custom property pass a CSS-wide keyword throughvar().What a flagged stylesheet changes. Two reads in
src/css/rules/mod.rs:merge_style_rulesreturns false. No rule merges into the rule before it, also not two rules with equal blocks.Every merge is off, not only the merge of unequal blocks. The others look safe and are not:
transition-property(css: compare custom property names inPropertyIdequality and importance in duplicate-rule detection #33684): main prints.a,.b{transition-property:fill}for.a{transition-property:fill}.b{transition-property:stroke}. A lonerevert-rulethere parses as a name.topwhen the repeat is removed from.a{top:6px}.a{inset-block-start:5px;top:revert-rule!important}.a{inset-block-start:5px;top:revert-rule!important}.Measurements. Release builds of
bun_cssin a small harness. Instruction counts areinstructions:ufromperf_event_open, minimum of 7 runs. "Base" is the stack base.Stylesheets without the keyword (15 files in
test/js/bun/css/files, 2.9 MB, 20,780 style rules):merge_style_rulescall. parse +6,670 of 521,224,514. Total +67,533 of 826,548,530 (+0.008%)..text+224 B.StylesheetExtragrows from 24 to 32 B, one per stylesheet.bun buildof the 15 files, with and without--minify, for--targetbrowser and bun: 60 of 60 outputs equal main's.Flagged stylesheets:
font-family:revert-ruleand are not flagged (css: keep css-wide keywords and percentage stretch out of the font and transition shorthands #42022)..zz{color:revert-rule}appended to each of 12,361 npm stylesheets: 3,220 change, +2,301,596 B of 493,364,183 (+0.467%), gzip -6 +83,224 B (+0.131%). The median file does not change, the 99th percentile is +6.9%, 58 files grow by more than 10%. The worst, a file of repeated rules, goes from 4,250 to 23,430 B..z{color:revert-rule}.a:not(.b,.c){color:red}has 4, 6, 8, 10, 12 and 14 rules after builds 1 to 6 (main: 4, then 6). With.a:autofill:not(.b,.c)it has 4, 8, 18, 42, 100 and 240 (main: 4, then 8).top,left,--xand--y. main gives another computed value for 65,442 of them, this PR for 0.WPT at 68d964a5f2, all 15 pages named
*revert-rule*(46 subtests). Each<style>block is built alone and put back:--minifygives the same counts. The subtest that fails after this PR and not before is "can revert from a CSSNestedDeclarationsRule" (#45462). Firefox has noif()and passes twoif()subtests by accident after a build. One style block (bare declarations in@scope) does not parse on any version and stays as written.Leftovers, with owners. The
test.todorows hold these. The PR that lands second turns its rows into tests.revert-ruleparses as a name in eight properties:animation,animation-name,font-family,transition,transition-property,view-transition-class,view-transition-group,view-transition-name.color-schemeprintsnormalfor it. The owner is css: keep css-wide keywords and percentage stretch out of the font and transition shorthands #42022. With it those values reach the token list and set the flag.inset-inline-start:revert-rulegoes to new:lang()rules, where it rolls back nothing. No owner.resultof a custom function (--f()). It can be an attribute throughattr(x type(*)): Firefox 157 and WebKit 27.2 take it, Chrome 153 with a leading space. It can be a name fromident("revert-" "rule"): WebKit 27.2 takes it. A flag for these would turn every merge off in each stylesheet that calls a custom function.@supportsrules with the same condition lose the second one, with or without the keyword: css: keep same-condition @supports rules and empty named @layer blocks #46733. css: parse (property: value) @supports conditions as declarations and merge same-condition rules #38589 joins them and then needs the same deferral as css: minify a run of merged same-query @media rules once #38531.allmoves in front of the declarations before it (css: make theallshorthand reset the declarations before it #42042). An unparsed value replaces a typed one before it (css: keep an unparsed width/height/color after the declarations written before it #42101). Where main's merge hid that, the output is now wrong in another way:.a{top:5px;all:unset}.a{top:revert-rule}.PropertyIdequality and importance in duplicate-rule detection #33684 and css: with browser targets, remove an earlier duplicate rule only when its declaration block is equal #46861. This PR prints main's output for those inputs. css: with browser targets, remove an earlier duplicate rule only when its declaration block is equal #46861 adds a conjunct to the sameif. The two are independent.Other designs
.a{top:1px}.a{inset-block-start:revert-rule}.Self-review, not done
test.failingin place oftest.todo. A fix that lands from an older branch would then turn main red.Suites run on the debug build.
test/js/bun/css/,test/bundler/css/,test/bundler/esbuild/css.test.ts,test/bundler/bundler_html.test.ts. The 4 failures are time-boxed tests incss-fuzz.test.ts. They fail the same way on main in a debug build.