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An import with the `type: "css"` attribute now evaluates to a constructed
CSSStyleSheet that holds the bundled CSS of that file, and the file no
longer lands in the importing entry point's stylesheet. Before, the JS
binding was `{}` and the rules were applied to the whole page.
The dev server cannot represent this yet and reports the import as
unsupported instead of emitting a dangling binding.
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Status Reproduced on 1.4.3-canary (f42e980) with the three-file page from the report ( bun build ./index.html --outdir out
# out/index-*.js: // widget.css
# var widget_default = {};
# out/index-*.css: p.w { ... } (linked from <head>)In headless Chrome the built page traced Tests: CI (build 113452 at 65e38f0, same tree as de5bb03): 181 of 182 jobs passed. The one red lane is |
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Updated 10:22 PM PT - Sep 8th, 2026
❌ @robobun, your commit 65e38f0 has 1 failures in 🧪 To try this PR locally: bunx bun-pr 41959That installs a local version of the PR into your bun-41959 --bun |
WalkthroughAdds browser CSS module script support. CSS imports with ChangesCSS module scripts
Suggested reviewers: Priority: ➖ Normal Merge Risk: 🔵 Low · up to Browser CSS module imports now produce constructed stylesheets, but named imports may still be accepted and yield undefined instead of a clear error. The new TypeScript declaration may also fail the package formatter until its syntax support or file coverage is adjusted. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
Warning Some tools did not complete. Review the errors below. 🔧 Biome (2.5.8)packages/bun-types/ts7.1/import-attributes.d.tsFile contains syntax errors that prevent linting: Line 17: Expected a semicolon or an implicit semicolon after a statement, but found none; Line 17: expected ... [truncated 684 characters] ... xpected a semicolon or an implicit semicolon after a statement, but found none; Line 70: expected Comment |
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Inline comments:
In `@src/bundler/linker_context/scanImportsAndExports.rs`:
- Around line 235-253: Update the CSS module script tracking and
chunk-generation logic around css_module_scripts and computeChunks.rs so the CSS
module script treatment is tied to the specific import record or dynamic entry
point, not only the target source_index. Ensure an attributed dynamic import
cannot cause an unrelated un-attributed import of the same stylesheet to use the
CSS module script stub or CSSStyleSheet output, and add regression coverage for
this mixed dynamic-import case.
In `@src/js_parser/p.rs`:
- Around line 1471-1477: Validate CSS module script import clauses in both
branches that assign CSS_MODULE_SCRIPT: reject every imported name except
"default" before completing the import record, matching the existing Sqlite and
File/Text validation behavior. Use the surrounding import-name validation
symbols and preserve the current loader and flag assignment for valid default
imports.
In `@test/bake/dev/css.test.ts`:
- Around line 723-725: Update the plain CSS import test around the dev.write
call and expectMessage("plain") so widget.css styles an existing element such as
body, then assert that element’s computed style after the message assertion,
verifying the stylesheet is registered and applied.
In `@test/bundler/css/css-module-scripts.test.ts`:
- Line 187: Replace the manual splitting loop with a
describe.each()-parameterized suite, preserving both false and true splitting
variants and the existing tests within each suite.
- Line 287: Add a parallel target: "node" regression case alongside the existing
Bun case, asserting the default export remains {} and generated JavaScript does
not construct CSSStyleSheet. Reuse the existing test setup and assertions where
possible, changing only the target-specific configuration.
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📒 Files selected for processing (16)
src/ast/import_record.rssrc/bundler/LinkerContext.rssrc/bundler/bundle_v2.rssrc/bundler/linker_context/computeChunks.rssrc/bundler/linker_context/findAllImportedPartsInJSOrder.rssrc/bundler/linker_context/findImportedCSSFilesInJSOrder.rssrc/bundler/linker_context/generateChunksInParallel.rssrc/bundler/linker_context/generateCodeForLazyExport.rssrc/bundler/linker_context/generateCompileResultForCssChunk.rssrc/bundler/linker_context/mergeSmallChunks.rssrc/bundler/linker_context/prepareCssAstsForChunk.rssrc/bundler/linker_context/scanImportsAndExports.rssrc/js_parser/p.rssrc/runtime.jstest/bake/dev/css.test.tstest/bundler/css/css-module-scripts.test.ts
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…ver export-from
- url() to a copied asset prints as ${__cssUrl(path, import.meta.url)} in
ESM output, so it resolves relative to the JS chunk (where the asset path
points) instead of the page.
- Go through globalThis for CSSStyleSheet and URL so bundling does not
reserve those names and rename user declarations (happy-dom's
CSSStyleSheet class).
- export { default as x } from "./a.css" with { type: "css" } (and other
type attributes on export-from) now applies the attribute's loader like
import does.
- Update the two debugId snapshots that move whenever runtime.js changes.
- Docs: CSS module scripts section in docs/bundler/css.mdx.
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Inline comments:
In `@src/js_parser/p.rs`:
- Around line 1468-1474: Update the Loader::Css assignment path near
set_import_record_loader to apply the existing default-only CSS module-script
validation before the import record reaches linking. Ensure named imports such
as sheet are rejected with a build error while the default-only import behavior
remains valid.
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docs/bundler/css.mdxsrc/ast/import_record.rssrc/bundler/LinkerContext.rssrc/bundler/bundle_v2.rssrc/bundler/linker_context/computeChunks.rssrc/bundler/linker_context/findImportedCSSFilesInJSOrder.rssrc/bundler/linker_context/generateChunksInParallel.rssrc/bundler/linker_context/generateCodeForLazyExport.rssrc/bundler/linker_context/generateCompileResultForCssChunk.rssrc/bundler/linker_context/scanImportsAndExports.rssrc/js_parser/p.rssrc/js_parser/parse/parse_stmt.rssrc/runtime.jstest/bake/dev/css.test.tstest/bundler/bundler_promiseall_deadcode.test.tstest/bundler/css/css-module-scripts.test.tstest/regression/issue/cyclic-imports-async-bundler.test.js
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Reject named imports with { type: "css" } at parse time, like the
file and text loaders do, instead of leaving it to the linker.
…chunked files - packages/bun-types/ts7.1/import-attributes.d.ts: a CSS module script is a CSSStyleSheet when lib.dom is loaded, else an object. - is_chunked_as_js(): the single place that decides a CSS module script's stub is placed like a JS file (computeChunks, findAllImportedPartsInJSOrder, mergeSmallChunks).
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Inline comments:
In `@packages/bun-types/ts7.1/import-attributes.d.ts`:
- Line 89: Update the bun-types formatting configuration so Biome no longer
rejects the TypeScript 7.1 import-attribute declaration in
import-attributes.d.ts: either pin Biome to a version supporting this syntax or
exclude that declaration from formatting. Preserve the existing type-check
workflow.
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packages/bun-types/ts7.1/import-attributes.d.tssrc/bundler/LinkerContext.rssrc/bundler/linker_context/computeChunks.rssrc/bundler/linker_context/findAllImportedPartsInJSOrder.rssrc/bundler/linker_context/mergeSmallChunks.rstest/bundler/css/css-module-scripts.test.tstest/integration/bun-types/fixture/ts7.1/import-attributes.ts
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2 participants
Problem
import sheet from "./widget.css" with { type: "css" }is a CSS module script: browsers evaluate it to a constructedCSSStyleSheetthat applies only where adopted.bun build --target=browser(also HTML entries,--splitting,--minify,--compile) emittedvar widget_default = {};and merged the rules into the page stylesheet.adoptedStyleSheets = [sheet]threw, and shadow-root rules styled the whole page.Loader::Css.findImportedCSSFilesInJSOrder.rspulls every JS-to-CSS edge into the entry's CSS chunk, andgenerateCodeForLazyExport.rsfills the JS stub with{}.Fix
CSS_MODULE_SCRIPT). For a browser-target importer the linker keeps the file out of the page CSS, and the stub exports__cssModule("<css>"), a new runtime helper:new CSSStyleSheet()plusreplaceSync().@imports inlined, minified when asked. Aurl()to a copied asset prints as${__cssUrl(path, import.meta.url)}in ESM output, so it resolves against the chunk, not the page. The stub is chunked as a JS module, so a sheet keeps one identity across chunks.bun/node(noCSSStyleSheet;bun rungives{}) and plain CSS imports. A.module.csswith the attribute is an error. The dev server reports the import as unsupported for now, instead ofReferenceError: import_widget is not defined.test/bundler/css/css-module-scripts.test.ts(14 of 16 fail on stock bun), a case intest/bake/dev/css.test.ts, thets7.1types fixture, the reported page in headless Chrome 152. Self-reviewed: 7 concerns raised, 6 addressed; the per-file model below stays until modules are keyed by (path, attributes).Background
{}, or the class map for CSS modules.Notes
["isCSSStyleSheet=true","adopted","shadow=rgb(200, 0, 0) light=rgb(0, 0, 0)"], identical to the unbundled page. Stock bun 1.4.3:["isCSSStyleSheet=false","adopt threw TypeError","shadow=rgb(0, 0, 0) light=rgb(200, 0, 0)"].<link rel="stylesheet">for it.LinkerContext::css_module_scriptsmaps the CSS source index to the string literal of the stub's call.scan_imports_and_exportsfills the map,generate_code_for_lazy_exportcreates the call with an empty literal, andgenerate_chunks_in_parallelprints the CSS into it before any JS is printed (aftermangle_local_css, with the same prepare and print steps a CSS chunk uses).url()asset (ESM):background: url("${/* @__PURE__ */ __cssUrl("./img-bz4ar6wy.png", import.meta.url)}");. Checked in headless Chrome with the page at/pages/deep/, the chunk at/js/, the asset at/assets/: computedbackground-imageis/assets/img-bz4ar6wy.pngand fetches with 200. TheCSSStyleSheetconstructor'sbaseURLoption would be simpler but Chrome 152 ignores it. IIFE and CJS output have noimport.meta, so there the path stays chunk-relative text, the same convention asimport png from "./a.png".CSSStyleSheetandURLare referenced throughglobalThisin the runtime. A bare reference reserves the name during renaming and turned happy-dom'sclass CSSStyleSheetintoCSSStyleSheet2in the bytecode portability corpus.import { x } from "./w.css" with { type: "css" }) is a parse error, like with thefileandtextloaders: a CSS module script only has a default export.export { default as sheet } from "./w.css" with { type: "css" }andexport * fromapply the attribute's loader too (before, type attributes onexport … fromwere dropped for every loader).packages/bun-types/ts7.1/import-attributes.d.tstypes the import asCSSStyleSheetwhen lib.dom is loaded, elseobject.import "./w.css"andimport s from "./w.css" with { type: "css" }) keeps both behaviors: it stays in the page CSS because of the plain import, andsis a sheet. A plainimport x from "./w.css"elsewhere in the same browser graph then also sees the sheet instead of{}. A server graph in the same build has its own copy of the file and is unaffected.import sheet from "./w.css" with { type: "css" }emits neither JS nor CSS (the__cssModule()call is marked pure).import("./w.css", { with: { type: "css" } }): with--splittingthe target becomes a JS chunk that exports the sheet. Without splitting it is the usual inlined lazy module. A plainimport("./w.css")in a browser build pointed at a.csschunk that browsers refuse to load as a module; when the file is also imported with the attribute it now resolves to the stylesheet module.replaceSync()ignores@import, so an external@importleft in the text is dropped by the browser, as in a native CSS module script. Bundled@imports are inlined.import()target.bun build --no-bundle/Bun.Transpilerstrip the attribute from the printed import for non-bun targets (the other half of Bun.build & Bun.Transpiler CSSStyleSheet loader #15310). The dev server needs CSS files to exist as modules in its client graph before it can export a live sheet; that is follow-up work, so it errors clearly for now.debugIdsnapshots (bundler_promiseall_deadcode.test.ts,cyclic-imports-async-bundler.test.js) move wheneverruntime.jschanges, as in bundler: read import() / require() exports without a namespace object #41186.test/bundler/esbuild/css.test.ts,test/bundler/css/css-modules.test.ts,test/bundler/bundler_html.test.ts,test/bundler/bundler_splitting.test.ts,test/bundler/esbuild/loader.test.ts,test/bundler/bundler_loader.test.ts,test/bundler/html-import-manifest.test.ts,test/bake/dev/css.test.ts.[human-review] gate passed · iteration 0 · 22 files touched
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