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Both are in css-view-transitions-2. The parser did not know them, so `bun build` printed a warning for valid CSS: warn: Invalid selector. Unsupported pseudo-class or pseudo-element 'active-view-transition-type' warn: invalid @ rule encountered: '@view-transition' Add `PseudoClass::ActiveViewTransition`, `PseudoClass::ActiveViewTransitionType` and `CssRule::ViewTransition` with the `navigation` and `types` descriptors. A descriptor that does not parse is kept as written. In a CSS module the view transition types are not hashed. Script passes the same names to `document.startViewTransition({ types })`.
Blink accepts any ident token in the argument, so a type named `default` or `initial` works in Chrome. A `<custom-ident>` parse made such a stylesheet fail the build. Hold the types as plain `Ident` values, which a CSS module does not rename. Also test that the printed text parses back to the same rules, and say why `@view-transition` keeps a descriptor that it cannot parse.
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Status Reproduced on 1.4.3 and on main: With this branch the build prints no warning. The new tests in The eight names of the same class that this PR leaves out are in #42909. |
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Included review availability: Your plan provides up to 10 included reviews per hour; 0 remain after this review. WalkthroughThe change adds CSS View Transition at-rule and pseudo-class support. It adds typed parsing, serialization, CSS Modules handling, validation, and bundler and minifier coverage. ChangesCSS View Transition Support
Suggested reviewers: Priority: ⬇️ Low Merge Risk: ⚪ Minimal · up to View-transition types remain compatible with names supplied through the browser API, and no actionable merge risk was identified. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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In `@test/bundler/css/view-transition-23600.test.ts`:
- Line 165: Replace the parameterized test declarations using test.each with
describe.each for the CSS cases, and place the asynchronous test body inside
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src/css/css_parser.rssrc/css/rules/mod.rssrc/css/rules/view_transition.rssrc/css/selectors/parser.rssrc/css/selectors/selector.rssrc/css/values/ident.rstest/bundler/css/css-modules.test.tstest/bundler/css/view-transition-23600.test.tstest/js/bun/css/css.test.ts
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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 nit, I also checked the warn-to-error change for @ view-transition nested in a style rule / with a prelude / as a statement and for non-ident :active-view-transition-type() arguments — each fails through the same parse_prelude / parse_comma_separated paths that @ font-face and :dir() already use, so the new errors are consistent with existing typed rules. The Custom fallback's ParserOptions::default(None) (which drops dashedIdents context) mirrors font_face.rs, container.rs and font_palette_values.rs, so it is a pre-existing pattern rather than a new gap. The CSS-modules decision to leave types unhashed is a design choice the author flagged for confirmation and still needs a human call.
Extended reasoning...
The diff adds a new @ view-transition rule file that copies the const _: () = { impl ... } parser-trait block from font_face.rs, wires it into AtRulePrelude and the css_rule_variants! table (the count comment 20 -> 21 is accurate), and adds two pseudo-classes with a serializer arm. deep_clone covers all three ViewTransitionProperty variants. The remaining open question is a product decision (whether CSS modules should hash view-transition type idents, and whether previously-warning inputs should now fail the build), which the PR description explicitly asks a maintainer to confirm, so this should not be auto-approved.
2 verified lower-impact observations (convention, logging or cleanup points) were not posted.
…input Use describe.each for the two case tables, check the message of each build error, and add the `@view-transition;` case.
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I reviewed the updated PR and didn't find any bugs; the error-message assertions and the @ view-transition; case from my earlier note are in as of b16693d. A maintainer should still weigh the two choices the description flags: not hashing view-transition types in CSS modules (diverges from lightningcss) and turning the previously warning-only inputs into build errors.
What was reviewed:
view_transition.rsagainstfont_palette_values.rs/font_face.rs: same RuleBodyParser loop, same state-reset fallthrough toCustomProperty, sameto_cssshape (thelen - 1is only reached inside a non-empty loop).CssRule::ViewTransitiongoes through thecss_rule_variants!macro (to_css/deep_clone generated);minifyfalls into the existing_ => {}arm;allowed_in_style_ruleintentionally excludes it, matching@ font-face.PseudoClassderivesClone, CssEql, CssHashandVec<T>impls exist for both, so the newVec<Ident>variant compares/hashes;Ident::to_csscallsserialize_identifierwith no module hashing, consistent with the newto_css_with_options(dest, false)path.
Extended reasoning...
Overview
The PR adds css-view-transitions-2 support: PseudoClass::ActiveViewTransition, PseudoClass::ActiveViewTransitionType { types: Vec<Ident> }, and a new CssRule::ViewTransition backed by src/css/rules/view_transition.rs, plus a NoneOrCustomIdentList::to_css_with_options overload so types: is printed without CSS-module hashing. Registration is one line in the css_rule_variants! table and one prelude arm in css_parser.rs. Tests were added to the existing view-transition-23600.test.ts, css-modules.test.ts, and css.test.ts. The final commit (b16693d) converted the rejection tests to describe.each with an expected-message column and added the @ view-transition; row, which addresses the inline nit from my previous review.
Security risks
None specific to this change. The parser only accepts idents via the existing tokenizer (expect_ident, CustomIdent::parse), and the fallback path reuses CustomProperty::parse exactly as @ font-face and @ font-palette-values do. No new allocation sizes derive from untrusted lengths beyond what the sibling rules already do. Error paths return Err through the existing css::Result machinery rather than panicking.
Level of scrutiny
Moderate. The native code is a close structural copy of font_palette_values.rs, and I diffed each piece against its sibling: the declaration parser (state save, typed parse + expect_exhausted, reset, CustomProperty::parse with ParserOptions::default(None)), the reject-all AtRuleParser/QualifiedRuleParser impls, the to_css loop, and deep_clone. PseudoClass derives Clone, CssEql, CssHash, and generics.rs has CssEql/CssHash impls for Vec<T>, so the new Vec<Ident> payload participates in equality and hashing without hand-written arms. CssRuleList::minify has a _ => {} arm so the new variant is preserved unchanged. AtRulePrelude::allowed_in_style_rule does not list ViewTransition, which is what produces the pinned "Unknown at-rule @ view-transition" error for nested use, consistent with @ font-face. The reason I am deferring rather than approving is not code risk but that the PR explicitly asks a maintainer to confirm two user-visible decisions: CSS modules leaving view-transition type names unhashed (lightningcss hashes them), and inputs that used to warn now failing the build.
Other factors
No CODEOWNER covers src/css/. Test coverage is reasonably thorough: minify/non-minify, plain and .module.css inputs, round-trip of the printed output, nested-in-@ media/@ layer, unparseable descriptors preserved verbatim, and exact error messages for eight rejected inputs. The bug-hunting run exited on dry_streak with no findings. Nothing in the timeline indicates an outstanding objection from a human reviewer.
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Updated 5:05 AM PT - Sep 16th, 2026
✅ @robobun, your commit 4ddd1b8ab4ceb39b9514028cee749e1ae40ba960 passed in 🧪 To try this PR locally: bunx bun-pr 42912That installs a local version of the PR into your bun-42912 --bun |
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Review follow-up:
Two choices still need a maintainer. Both are in the description: a CSS module does not hash view transition types, and input that no browser takes now fails the build. |
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Follow-up to #42777 and #42779: two warning-only cases. Eight more names of this class are not here (#42909).
Problem
bun buildwarns on two valid css-view-transitions-2 constructs:warn: Invalid selector. Unsupported pseudo-class or pseudo-element 'active-view-transition-type'andwarn: invalid @ rule encountered: '@view-transition'. The output is correct.parse_non_ts_functional_pseudo_class(src/css/selectors/parser.rs:1342) andparse_prelude(src/css/css_parser.rs:1522) have no arm for them. The unknown path warns and keeps raw tokens.Fix
PseudoClass::ActiveViewTransition,PseudoClass::ActiveViewTransitionTypeandCssRule::ViewTransition(src/css/rules/view_transition.rs) withnavigationandtypes, as in lightningcss. The pseudo-class takes any ident, as Blink does. A descriptor that does not parse is kept as written.startViewTransition({ types })) and Bun exports only classes and ids. Please confirm this choice.(),(*),("a")),@view-transition;and@view-transition foo {}warned before. Now they fail the build, like:dir(sideways).test/bundler/css/view-transition-23600.test.ts(12 new tests fail on the released bun),css-modules.test.ts,css.test.ts,test/bundler/css/. Self-reviewed: see Notes.Background
@view-transition { types: a b }for a cross-document navigation.:active-view-transition-type(a, b)matches the root element while one of them is active.CustomIdent::to_cssadds a file hash.Ident::to_cssandto_css_with_options(dest, false)do not.Notes
Repro, before and after:
Output changes. The text now goes through the printer and not through the raw token list:
:active-view-transition-type(slide-in, reverse), minified(slide-in, reverse)(slide-in,reverse)@view-transition { navigation: auto; types: foo bar }, minified@view-transition{navigation: auto; types: foo bar}@view-transition{navigation:auto;types:foo bar};:ACTIVE-VIEW-TRANSITION,@VIEW-TRANSITION,NAVIGATION: NONEInput that fails the build now. It built with a warning before. The messages are the ones that the other typed selectors and at-rules give:
:active-view-transition-type(),(a,)Unexpected end of input(a b),(*),("a"),(1)Unexpected token: b(and*,"a",1)a { @view-transition { ... } }Unknown at-rule @view-transition(the same as@font-facein a style rule)@view-transition foo { ... }Unexpected token: foo@view-transition;Unexpected token: ;Blink, Gecko and WebKit reject each of these, so the rule was dead in the browser. Inside
:is()and:where()the parser drops only the bad selector, with no log entry, as it does for:is(:dir(sideways), .x).Any ident in the pseudo-class. The spec grammar is
<custom-ident>#. Gecko, WebKit and lightningcss parse a custom ident, which rejectsdefaultand the CSS-wide keywords. Blink takes any ident token (kPseudoActiveViewTransitionTypein css_selector_parser.cc). A type nameddefaultorinitialworks in Chrome, so a custom ident parse would fail a build that works today. The variant holdsVec<Ident>, like::picker().Ident::to_cssnever adds a module hash.Descriptors.
navigationisauto | none.typesisnone | <custom-ident>+and uses the existingNoneOrCustomIdentList(the type ofview-transition-class), printed through the newto_css_with_options(dest, false). lightningcss drops a descriptor with an unknown name. This change keeps it as aCustomProperty, so a descriptor from a later spec level is not lost.navigation: sideways,types: a, b,types: defaultandfuture-descriptor: 1 2print as written. The rule is accepted at the top level and inside@media,@supports,@layer,@containerand@scope, as in lightningcss.minifydoes not merge or drop it.CSS modules. The types in
:active-view-transition-type()and intypes:must agree, so both are hashed or neither is. Reasons for neither:document.startViewTransition({ update, types: ["forwards"] }), orevent.viewTransition.types.add()in apagerevealhandler. The exports object in Bun has class and id symbols only (view-transition-name: herois hashed and is not exported). Script could not get the hashed type.forwardsmust see the same name.view-transition-nameis different: it must be unique in the document, so a hash per file helps.css-module/ViewTransitionTypesNotScopedpins this. The first new test inview-transition-23600.test.tsruns onin.cssand onin.module.css, expects the same text, then builds the printed text again and expects the same text once more.Not in this PR. The same class of false warning remains for eight names that lightningcss parses:
:state(),::highlight(),::target-text,::search-text,::spelling-error,::grammar-error,@position-tryand@font-feature-values. They are left out on purpose. Two of them need the same CSS module decision, and the two at-rules are each a new rule file. #42909 tracks them. It also has the wider question: Bun prints these fallback warnings always, and the lightningcss CLI prints them only with--error-recovery. #41120 and #42480 track the names that lightningcss does not know either.The parser trait impls.
view_transition.rshas the same 51 lines of reject-allAtRuleParserandQualifiedRuleParserimpls that each declaration parser has. #31999 (open) gives these traits default bodies. The PR that lands second drops the lines.Upstream tests. The new
minify_testcases incss.test.tsare the lightningcss cases for:active-view-transition,:active-view-transition-type()andtest_view_transition.Open PRs in this area. A trial merge of this branch with #42764 and with #38572 has no conflict. #38572 names
:active-view-transitionas a false positive that it would add. This change removes that case.Self-review. A review of the diff before this PR kept it as one PR and asked for these changes. All are in: the comment on the kept descriptors, the list of names that are left out with a tracker, the #31999 link, the list of input that fails the build now, and the request to confirm the CSS module choice. The any-ident grammar and the test that builds the printed text again also came out of that review. It had compared the argument grammar with the Blink source. One suggestion is not taken: to split the selector half from the at-rule half. The two halves are one feature and the selector half is 24 lines. If the at-rule draws design debate, the selector half can land first on its own.
Suites, debug + ASAN build:
test/bundler/css/andtest/bundler/esbuild/css.test.ts248 pass, 0 fail.test/js/bun/css/css.test.ts1188 pass. In the rest oftest/js/bun/css/, tests incss-fuzz.test.tsandnested-selector-expansion.test.tshit their 5 s and 10 s limits under the debug build. They are bounded-time tests with no view transition input.cargo clippy -p bun_cssis clean.[human-review] gate passed · iteration 1 · 9 files touched
fails on main (without fix)
passes on PR (with fix)
diff hotspot
gate history · 1 passed · 1 rejected · iteration 1
evidence per changed file