[robobun]

Problem

• A CSS module prints @keyframes spin as spin_BLNoTg, but its exports object has no spin key. import {spin} fails with No matching export in "a.module.css" for import "spin".
• The object comes from local_scope (src/bundler/linker_context/generateCodeForLazyExport.rs:81), which only class and id selectors fill. Printer::write_ident (src/css/printer.rs:559) renames a keyframes name at print.

Fix

• KeyframesName and AnimationName get a Local variant that holds a symbol. The parser sets it when the rule or declaration list is accepted. The printer prints it like a class.
• Correct because CSS and JS read one symbol. esbuild, Lightning CSS and postcss-modules export these names too.
• Verified: test/bundler/css/css-modules.test.ts (14 tests fail on main), test/bundler/esbuild/css.test.ts (1). Also test/bundler/css/, test/js/bun/css/.
• Self-reviewed: 17 concerns raised, 15 addressed. Notes name the two not taken.

Background

• A symbol is an entry in a file's symbol table. The linker names each local symbol. The exports object has one key per symbol.
• The value parsers run speculatively, so a name is final only when its list is.
• Considered: add the key and keep the rename at print. Two name generators must then agree.

Downsides

• A keyframes-only module exports {k: "k_<hash>"}. bun test: Vitest compatibility #44833 expects {}. Question for @Jarred-Sumner: is that key wanted?
• A keyframes name costs what a class name costs: about 20,000 instructions and 20 allocator calls in a bundle. A module with no keyframes pays 95 instructions per rule. .text grows 2,304 bytes.
• @keyframes await with a named import gives a bundle that does not load. A class .await does the same on main (bundler: reserve await, eval and arguments in esm output and for bundler-made bindings #34361).

Notes

Origin. No issue reports this. A comment on #18921 asks for the keyframes name in the exports object, to compare it with event.animationName. #42729 fixed the same kind of bug (a name that is renamed but not exported) for the class in ::view-transition-*(.x), also with a symbol.

The question about #44833

• bun test: Vitest compatibility #44833 builds the runtime object from local_scope, so it gets the new keys with no code change.
• Three expectations in its test/js/bun/css/css-loader.test.ts gain a key:
  • "a module without classes or ids" expects noLocals: {}. It becomes { k: "k_<hash>" }.
  • "classes and ids, wherever they appear" gains spin.
  • The global-keyframes.module.css case gains spin.
• If the key is wanted, the second of the two PRs to land updates these three objects.
• A trial merge with its head 2705a15 conflicts with this PR only in docs/bundler/css.mdx and test/bundler/css/css-modules.test.ts.

Which hunks are the fix

• The names: KeyframesName::Local(Ref) (src/css/rules/keyframes.rs) and AnimationName::Local { ref_, quoted } (src/css/properties/animation.rs). quoted keeps spin and "spin" unequal, as on main, so no rule merges that did not merge before.
• The symbol: Parser::add_symbol_for_keyframes_name (src/css/css_parser.rs). make_local calls it at five places:
  • the end of DeclarationBlock::parse, of NestedRuleParser::parse_nested and of PageRule::parse
  • the @keyframes arm
  • the @container arm, for style() queries
• The print: Printer::write_local_symbol (src/css/printer.rs). In a bundle it reads the name that the linker made. bun build --no-bundle has no link step, so there it hashes the original name, as css: export view-transition pseudo-element classes from CSS modules #42729 does for a class.
• Nothing in bun_bundler changes. The linker already names each LocalCss symbol and builds the object from local_scope.
• Removed: CssModule::get_reference and Config.animation. No reader is left.

What changes for users

• styles.spin is spin_<hash>. import { spin } builds. A name that is not a JS identifier (fade-in) is a key of the default object only.
• Every keyframes name that the parser accepts as a local name gets a key. A reference with no rule in the file gets one too: .a { animation-name: fade } exports fade. The CSS already renamed it. esbuild 0.28.1, Lightning CSS 1.33.0 and postcss-modules 9.0.1 do the same.
• A rule that the minifier drops keeps its keys. @media not all { .z { animation-name: zz } } exports z and zz, and the CSS has neither. Main does this for z. esbuild exports both names too.
• A class or an id with the same name shares the key. composes reads the class. An empty class (.spin {}) gave the key before. That still works and gives the same value.
• A name that the CSS does not rename gets no key:
  • @keyframes "none"
  • a value that stays raw tokens (animation: fade var(--d))
  • a declaration of a dropped keyframe
  • animation-name:hover {}, which is a nested rule
• Key order: a name appears when its rule or its declaration list closes. @keyframes outer { to { animation-name: inner } } gives { inner, outer }, as in esbuild. In one list, the names of !important declarations come after the others. esbuild keeps source order there.
• composes: spin for a name that is only a keyframes name is still a build error. The message was The name "spin" never appears in "a.module.css" as a CSS modules locally scoped class name. It is now The composes property cannot be used with "spin", because it is not a single class name., with a note at the name.

An animation name has no opt-out

• An animation name in a module always means keyframes of that module. Main has this rule since css: scope animation-name to its @keyframes hash in CSS modules (#18921) #33322. This PR adds the key.
• A module that names keyframes of another stylesheet gets a key that names no rule.
• The docs give the two ways that work. For keyframes of another CSS module, use the identifier from the object of that module. For keyframes of a plain stylesheet, pass the name through a custom property: --name: fade; animation: var(--name) 1s prints as written.
• :global(div) { animation-name: x } renames x, as in esbuild. @keyframes :global(x) is a build error.

CSS output. Release builds of main 5c6d4e2 and of this change on that commit, compared byte for byte.

• 598 builds: 122 inputs as bundle, --minify, --splitting, --no-bundle and --no-bundle --minify. 586 are equal. Exit codes and diagnostics are equal in all 598.
• The 12 that differ are 4 inputs in the 3 bundle modes. Each has a keyframes name that is -. It prints -_<hash>, and main prints \-_<hash>. Both spell the same identifier. Main prints the class .\- as .-_<hash> too.
• 15 stylesheets of test/js/bun/css/files and 1 file of keyframes shapes, as modules and as plain CSS: 80 files, 0 differ.
• The objects of those modules gain 294 keys. Examples: animate.css 97, uikit.css 20, materialize.css 11, bulma.css 3, bootstrap-4.css 1. No key changes its value or its position.

Not changed

• The same kind of bug, and this mechanism fits:
  • @container names with container-name (css: scope container and container-name to the @container hash in CSS modules #45525 and fix(css): keep container names global in CSS modules #45247 disagree on the rename)
  • @counter-style names (css: print the space after @counter-style, and hash the counter style name in list-style in CSS modules #46692)
  • view-transition-name, view-transition-group, the view-transition-class value and the name in ::view-transition-*(name)
• The same kind of bug, and this mechanism does not fit: env(<ident>) and the <custom-ident> values of @property. There the rename is the defect.
• Those names still go through Printer::write_ident(name, true).
• A name inside :global(...) still gets a key: css modules: decide local vs global at parse, fixing :global() exports and & inside :global() #45487.
• await, eval and arguments: for a named import of that key, or a namespace import that escapes, the bundler prints var await = "await_<hash>". The bundle does not load. A class or an id with that name does the same on main. For a keyframes name it is new: on main the named import is a build error. bundler: reserve await, eval and arguments in esm output and for bundler-made bindings #34361 reserves the three names.

The two concerns not taken

• mangle_local_css makes one arena and hashes the file path again for each local name (src/bundler/LinkerContext.rs:2631). That is 13,400 of the 20,000 instructions for a name (gdb single-step, slope from 40 to 80 names). A hoist of the arena is another change, and bun test: Vitest compatibility #44833 edits the same loop. bundler: append a CSS module's exports without a lookup per name #45824 also names it as not owned.
• A guard test that lists each renamed name with no key. css: scope container and container-name to the @container hash in CSS modules #45525, fix(css): keep container names global in CSS modules #45247 and css: print the space after @counter-style, and hash the counter style name in list-style in CSS modules #46692 change that list.

Order with open PRs

• css modules: tag the keyframes name when an animation value falls back to raw tokens #46034 tags a keyframes name in a raw-token animation value. After this PR it needs an arm for Property::Unparsed in animation::make_names_local, or the tagged name prints as written. It conflicts in src/css/properties/animation.rs.
• css: keep declarations written after a nested rule in place with a nested declarations rule #45462 puts a declaration that follows a nested rule in a new list. That list needs the make_animation_names_local call. No conflict marker shows this. css-module/KeyframesNamesInEveryDeclarationList fails without the call, because it has animation after a nested rule.
• css: scope container and container-name to the @container hash in CSS modules #45525 or fix(css): keep container names global in CSS modules #45247: container names depend on that ruling. If they stay renamed, make_local gives them a key. css: scope container and container-name to the @container hash in CSS modules #45525 conflicts in src/css/css_modules.rs.
• css modules: decide local vs global at parse, fixing :global() exports and & inside :global() #45487 conflicts in src/css/printer.rs and src/css/css_parser.rs. bundler: append a CSS module's exports without a lookup per name #45824 needs no change. css: print the space after @counter-style, and hash the counter style name in list-style in CSS modules #46692 conflicts only in the test file.
• CustomIdent::to_css_with_options keeps its enabled_css_modules parameter. This PR removes the only caller on main that passes false. fix(css): keep container names global in CSS modules #45247, css: scope container and container-name to the @container hash in CSS modules #45525 and css modules: decide local vs global at parse, fixing :global() exports and & inside :global() #45487 add callers that pass false or a variable, and fix(css): keep container names global in CSS modules #45247 merges with this branch with no text conflict.

Measurements. Release builds (bun run build:release, linux-x64) of main 5c6d4e2 and of this change on that commit. valgrind, perf, strace and bloaty are not installed. Instructions come from a small perf_event_open wrapper (user instructions of the process and its threads). Call counts come from gdb breakpoints. Sizes come from size -A.

• Binary: .text 82,231,705 to 82,234,009 bytes (+2,304). .rodata, .data and .bss are equal. File 107,808,480 to 107,812,576 bytes.

• Type sizes (-Zprint-type-sizes): no type of bun_css changes its size. Property is 216 B, AnimationName 24 B, KeyframesName 24 B, Animation 96 B, Printer 496 B.

• Plain CSS, for each declaration: +0. parse_declaration_impl, Property::parse, Property::to_css and both copies of RuleBodyParser::next are byte-identical to main. objdump shows 53 of 92,703 function bodies that differ.

• Plain CSS, for each declaration list: +3 instructions in DeclarationBlock::parse and +4 in parse_nested (gdb single-step of one call). That is the test of the module flag.

• Plain CSS end to end (parse, minify and print in one process), instructions for each item, slope from 20,000 to 60,000 items:

  Item	main	this PR
  style rule	9,458	9,436
  style rule with animation	14,882	14,884
  keyframe block	7,611	7,621
  @keyframes rule	3,659	3,651
  @page rule	9,553	9,572
  @container rule	7,564	7,576
  declaration	4,099	4,109

• Module with no keyframes, bun build --no-bundle (one thread, runs differ by less than 700 instructions):

  • 200 classes: 8,508,675 to 8,527,000 instructions.
  • 1,100 classes: 98,167,968 to 98,273,329.
  • That is 92 to 96 instructions for each rule (0.1% to 0.2%): the pass over each finished declaration list. Allocator calls are equal.

• Module with no keyframes, bundle: allocator calls are equal (16,213 for 200 classes). Instructions are inside the spread of two runs of one binary, about 0.3 M:

  • tailwind.css: 79.36 M and 79.88 M.
  • 4,000 classes: 834.13 M and 834.56 M.

• Each keyframes name in a bundle: +20 allocator calls and one mi_heap_new. A module with 100 names goes from 8.97 M to 11.01 M instructions, about 20,000 for each name. A class name costs the same on main.

• animate.css as a module (97 names): 26.38 M to 28.37 M instructions, and 6,104 to 9,679 bytes of JS.

• The cost of a name grows with the count of names in the file, because E::Object::put searches the earlier keys. 2,000 keyframes names beside 4,000 classes: 374.4 M to 623.6 M instructions. bundler: append a CSS module's exports without a lookup per name #45824 removes that search.

• With the count of keys held equal: 11.75 M and 11.49 M instructions. The fixture has 100 names that main exports through classes of the same name.

• Each keyframes name under --no-bundle: +830 instructions and +1.2 allocator calls.

• Print in a bundle, 100 rules with 100 references: 200 Printer::write_ident calls and 100 CssModule::get_reference calls on main, 0 here. Under --no-bundle the count of write_ident calls is equal (301).

• JS: 2 × length + 15 bytes for each name.

Tests

• css-module/NoBundleKeyframesNamesMatchBundle passes on main too. It holds the --no-bundle names equal to the names of a bundle.
• The new cases do not use require() of a CSS module: on main it returns {}.
• css/ImportCSSFromJSLocalAtKeyframes is the esbuild test of the same name, without the :global and :local blocks that Bun does not parse.

[robobun]

^{Updated 4:39 PM PT - Oct 24th, 2026}

✅ @robobun, your commit 1514c361bd44a6ee4ce7df6abfcb8ff6fa8a42f0 passed in Build #132240! 🎉

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bunx bun-pr 46720

That installs a local version of the PR into your bun-46720 executable, so you can run:

bun-46720 --bun

[robobun]

Status

Closed. The change landed in #47025 (7942008), in a smaller form.

Checked on main 7942008 (debug build, linux-x64)

printf '@keyframes spin{to{opacity:0}} .y{animation-name:spin}\n' > a.module.css
printf 'import s from "./a.module.css";\nconsole.log(s);\n' > e.js
bun build e.js --outdir=o && bun o/e.js

• The object is { spin: "spin_BLNoTg", y: "y_BLNoTg" }, and import { spin } builds. test/bundler/css/css-modules.test.ts passes (46 tests).
• css: consolidate the open CSS parser fixes #47025 adds the key for each @keyframes <ident> rule and keeps the hash at print time. It does not take the symbol variants of this PR.
• Two cases of this PR are not on main: a key for a quoted name (@keyframes "spin", listed as known in css: consolidate the open CSS parser fixes #47025), and a key for an animation name with no rule in the file.
• Container names are global in a CSS module now (css: consolidate the open CSS parser fixes #47025), so they need no key.

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Walkthrough

CSS Modules now localize keyframe names and animation references. The parser registers keyframe symbols, and the printer serializes them through symbol lookup. Tests cover CSS output, JavaScript exports, and parsing edge cases. The documentation describes keyframe exports and cross-stylesheet references.

Changes

CSS Modules keyframe localization

Layer / File(s)	Summary
Local symbol representation and serialization src/css/properties/animation.rs, src/css/rules/keyframes.rs, src/css/printer.rs	Animation and keyframe names gain local-symbol variants and conversion methods. The printer serializes local references through symbol lookup.
Parser localization and symbol registration src/css/css_modules.rs, src/css/css_parser.rs, src/css/declaration.rs, src/css/rules/container.rs, src/css/rules/page.rs	Parsing registers keyframe symbols and localizes animation names in declarations, keyframes, container conditions, and page rules. The animation configuration option and reference helper are removed.
Coverage and documentation docs/bundler/css.mdx, test/bundler/css/css-modules.test.ts, test/bundler/esbuild/css.test.ts	Tests cover keyframe exports, animation rewriting, parsing edge cases, and diagnostics. The documentation describes exported keyframe identifiers and cross-stylesheet references.

Suggested reviewers: jarred-sumner, alii

Priority: Normal

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No blocking behavior was established; the change is ready for normal merge checks.

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Check name	Status	Explanation
Title check		The title clearly and concisely identifies the main change: exporting CSS Module keyframe and animation names as symbols.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Description check		The description is complete and provides the problem, implementation, scope, verification details, performance measurements, compatibility notes, and known open questions. It does not use the template…

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Review comments at @docs/bundler/css.mdx:
- Line 953: Update the CSS Modules documentation paragraph about using keyframes
from another stylesheet to clarify that the custom property must be set from
JavaScript to the exported scoped identifier, such as `otherStyles.spin`, before
referencing it with `animation-name: var(--name)`.

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[claude]

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: every match on AnimationName (eql, hash, deep_clone, Animation::to_css keyword checks) handles the new Local variant and KeyframesName has no eql/hash pair to update; rerouting write_ident's symbol branch through write_local_symbol is behavior-preserving (the local_names path previously reached the same lookup_symbol via lookup_ident_or_ref); and the i32::try_from(start).expect("int cast") in add_symbol_for_keyframes_name matches how every other Loc in css_parser.rs is built, so it is not a new panic surface.

Extended reasoning...

The change moves CSS-module scoping of @ keyframes and animation names from print time to parse time by adding symbol-holding variants to KeyframesName and AnimationName, wiring make_local hooks at five rule-acceptance sites, and adding Printer::write_local_symbol; it touches no security-sensitive surface. It is a design-level change to what a CSS module exports (an open question to the maintainer about #44833 is in the description) and one confirmed finding records a build-error-to-runtime-failure regression for reserved JS names, so a human should weigh it rather than this review approving.

[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

Beyond the inline nit, I also swept for declaration-list producers the new make_animation_names_local hook could miss: every DeclarationBlock::parse caller (keyframe blocks, @ page margin rules, style attributes, the nested-rule path in css_parser.rs) plus @ page and @ container style() go through it, so no sibling site is skipped. The new Local variants are handled in eql, hash, deep_clone and to_css of both AnimationName and KeyframesName, and the earlier test.concurrent note is addressed in the current revision.

Extended reasoning...

The change moves CSS-modules keyframes/animation-name scoping from print time into parse-time symbols across src/css (parser, declaration, printer, animation, keyframes, container, page rules) with tests and docs; it touches no auth, crypto or injection surface. It is a non-trivial design change to the CSS modules export surface (new exported keys, open question to the maintainer about #44833) so a human should still weigh in; the only confirmed finding is a dead-parameter nit.

[Jarred-Sumner]

Landed in #47025 (7942008).

A CSS module exports its @keyframes names (styles.spin). Hashing is unchanged. 145 real stylesheets