CSS Import tests

@import in used as a signal by build tools to bundle CSS.
It is however important in my opinion that this is done in ways that match native browser behavior.

Without this parallel users of such tools become dependent on the tool itself, and the tool becomes a barrier to entry for new users.

When implementations differ between browsers and tools there is an increased chance that implementations will differ between multiple tools as well. This also blurs the line between feature and bug.

These tests are intended to surface such differences.

Currently this checks :

• chrome
• firefox
• esbuild
• lightningcss (abbr. ⚡️)
• @csstools/postcss-bundler (abbr. p-bundler)
• postcss-import (abbr. p-import)

bun was previously tracked here but was removed after it was rewritten in rust with an LLM

Configs

esbuild

No config needed to make it compliant.

const esBundle = await esbuild.build({
	entryPoints: ['style.css'],
	bundle: true,
	logLevel: 'silent',
	write: false,
	external: ['*.png'] /* for sub-resource tests */,
})

lightningcss

Some config is needed to make it run without fatal errors and make it more compliant.

let { code } = lightningcss({
	filename: 'style.css',
	errorRecovery: true, /* The CSS language is intended to be forgiving */
	resolver: { /* Adding this significantly slows down lightningcss */
		read(filePath) {
			return fsSync.readFileSync(filePath, 'utf8');
		},
		resolve(specifier, from) {
			if (/^https?:/.test(specifier)) {
				return { external: specifier }; /* imports with a full url must not be dropped from bundles */
			}
			return path.resolve(path.dirname(from), specifier);
		}
	}
});

@csstools/postcss-bundler

No config needed to make it compliant.

postcssBundler()

postcss-import

Some config is needed to make it more compliant.

postcssImport({
	skipDuplicates: false /* skipping duplicates is not how CSS works */,
})

How to run

Full test suite

• npm ci
• npm run install:with-firefox
• npm run test

Individual test

• npm ci
• npm run serve <name of a test>

Sub string matches are fine,
npm run serve at-layer will serve the first test that contains at-layer.

Current state

Test	chrome	firefox	webkit	esbuild	⚡️	p-bundler	p-import
001-core-features/001/absolute-url	✅	✅	✅	✅	✅	✅	✅
001-core-features/001/default	✅	✅	✅	✅	✅	✅	✅
001-core-features/001/foldername-that-is-a-domain	✅	✅	✅	✅	✅	✅	✅
001-core-features/001/relative-url	✅	✅	✅	✅	✅	✅	✅
001-core-features/at-charset/001	✅	✅	✅	✅	✅	✅	✅
001-core-features/at-charset/002	✅	✅	✅	✅	✅	✅	❌
001-core-features/at-keyframes/001	✅	✅	✅	✅	✅	✅	✅
001-core-features/before-other-styles/001	✅	✅	✅	✅	✅	✅	✅
001-core-features/before-other-styles/002	✅	✅	✅	✅	✅	✅	✅
001-core-features/byte-order-mark/001	✅	✅	✅	✅	❌	✅	✅
001-core-features/byte-order-mark/002	✅	✅	✅	✅	❌	✅	✅
001-core-features/byte-order-mark/003	✅	✅	✅	✅	❌	✅	✅
001-core-features/case-sensitivity/001	✅	✅	✅	✅	✅	✅	❌
001-core-features/case-sensitivity/002	✅	✅	✅	✅	✅	✅	❌
001-core-features/case-sensitivity/003	✅	✅	✅	✅	✅	✅	✅
001-core-features/cycles/001	✅	✅	✅	✅	✅	✅	✅
001-core-features/cycles/002	✅	✅	✅	✅	❌	✅	✅
001-core-features/cycles/003	✅	✅	✅	✅	❌	✅	✅
001-core-features/cycles/004	✅	✅	✅	✅	✅	✅	✅
001-core-features/cycles/005	✅	✅	✅	✅	✅	✅	✅
001-core-features/cycles/006	✅	❌	✅	✅	✅	✅	✅
001-core-features/duplicates/001	✅	✅	✅	✅	✅	✅	✅
001-core-features/duplicates/002	✅	✅	✅	✅	✅	✅	✅
001-core-features/empty/001	✅	✅	✅	✅	✅	✅	✅
001-core-features/escape-sequences/001	✅	✅	✅	✅	✅	✅	✅
001-core-features/escape-sequences/002	✅	✅	✅	✅	✅	✅	✅
001-core-features/escape-sequences/003	✅	✅	✅	✅	✅	✅	❌
001-core-features/escape-sequences/004	✅	✅	✅	✅	✅	✅	❌
001-core-features/escape-sequences/005	✅	✅	✅	✅	✅	✅	❌
001-core-features/forwards-compat/001	✅	✅	✅	✅	✅	✅	✅
001-core-features/forwards-compat/002	✅	✅	✅	✅	✅	✅	✅
001-core-features/forwards-compat/003	✅	✅	✅	✅	✅	✅	✅
001-core-features/forwards-compat/004	❌	✅	✅	✅	✅	✅	❌
001-core-features/forwards-compat/005	✅	✅	✅	✅	❌	✅	✅
001-core-features/forwards-compat/006	✅	✅	✅	❌	❌	✅	✅
001-core-features/forwards-compat/007	✅	✅	✅	✅	✅	✅	✅
001-core-features/forwards-compat/008	✅	✅	✅	❌	✅	✅	✅
001-core-features/input-preprocessing/001	✅	✅	✅	✅	✅	✅	❌
001-core-features/input-preprocessing/002	✅	✅	✅	✅	✅	✅	❌
001-core-features/mixed-importables/001	✅	✅	✅	❌	❌	✅	❌
001-core-features/namespace/001	✅	✅	✅	✅	✅	✅	✅
001-core-features/namespace/002	✅	✅	✅	✅	✅	✅	✅
001-core-features/relative-paths/001	✅	✅	✅	✅	✅	✅	✅
001-core-features/relative-paths/002	✅	✅	✅	✅	✅	✅	✅
001-core-features/subresource/001	✅	✅	✅	❌	❌	✅	❌
001-core-features/subresource/002	✅	✅	✅	✅	✅	✅	✅
001-core-features/subresource/003	✅	✅	✅	✅	✅	✅	✅
001-core-features/subresource/004	✅	✅	✅	❌	❌	✅	❌
001-core-features/subresource/005	✅	✅	✅	❌	❌	✅	❌
001-core-features/subresource/006	✅	✅	✅	✅	✅	✅	✅
001-core-features/subresource/007	✅	✅	✅	✅	✅	✅	✅
001-core-features/subresource/008	✅	✅	✅	❌	❌	✅	❌
001-core-features/subresource/009	✅	✅	✅	❌	❌	✅	❌
001-core-features/subresource/010	✅	❌	✅	✅	✅	✅	✅
001-core-features/subresource/011	✅	❌	✅	✅	✅	✅	✅
001-core-features/url-format/001/absolute-url	✅	✅	✅	✅	✅	✅	✅
001-core-features/url-format/001/default	✅	✅	✅	✅	✅	✅	✅
001-core-features/url-format/001/relative-url	✅	✅	✅	✅	✅	✅	✅
001-core-features/url-format/002/absolute-url	✅	✅	✅	✅	✅	✅	✅
001-core-features/url-format/002/default	✅	✅	✅	✅	✅	✅	✅
001-core-features/url-format/002/relative-url	✅	✅	✅	✅	✅	✅	✅
001-core-features/url-fragments/001	✅	✅	✅	✅	❌	✅	❌
001-core-features/url-fragments/002	✅	✅	✅	✅	❌	✅	❌
001-core-features/url-fragments/003	✅	✅	✅	✅	❌	✅	❌
001-core-features/url-fragments/004	✅	✅	✅	❌	❌	✅	❌
001-core-features/url-fragments/005	✅	✅	✅	❌	❌	✅	❌
001-core-features/url-fragments/006	✅	✅	✅	❌	✅	✅	✅
002-sub-features/001-data-urls/001	✅	✅	✅	✅	❌	✅	✅
002-sub-features/001-data-urls/002	✅	✅	✅	✅	❌	✅	✅
002-sub-features/001-data-urls/003	✅	✅	✅	✅	❌	✅	✅
002-sub-features/001-data-urls/004	✅	✅	✅	❌	❌	✅	❌
002-sub-features/001-data-urls/005	✅	✅	✅	✅	❌	✅	✅
002-sub-features/001-data-urls/006	✅	✅	✅	✅	❌	✅	✅
002-sub-features/002-at-media/001/absolute-url	✅	✅	✅	✅	✅	✅	✅
002-sub-features/002-at-media/001/default	✅	✅	✅	✅	✅	✅	✅
002-sub-features/002-at-media/002	✅	✅	✅	✅	✅	✅	✅
002-sub-features/002-at-media/003	✅	✅	✅	✅	✅	✅	✅
002-sub-features/002-at-media/004	✅	✅	✅	✅	✅	✅	✅
002-sub-features/002-at-media/005	✅	✅	✅	✅	✅	✅	✅
002-sub-features/002-at-media/006	✅	✅	✅	✅	✅	✅	✅
002-sub-features/002-at-media/007	✅	✅	✅	✅	✅	✅	✅
002-sub-features/002-at-media/008	✅	✅	✅	✅	✅	✅	✅
002-sub-features/002-at-media/009	✅	✅	✅	✅	✅	✅	✅
002-sub-features/002-at-media/010	✅	✅	✅	✅	❌	✅	✅
002-sub-features/002-at-media/011	✅	✅	✅	✅	❌	✅	✅
002-sub-features/002-at-media/012	✅	✅	✅	✅	❌	✅	✅
002-sub-features/002-at-media/013	✅	✅	✅	✅	❌	✅	✅
002-sub-features/002-at-media/014	✅	✅	✅	✅	❌	✅	✅
002-sub-features/002-at-media/015	✅	✅	✅	✅	✅	✅	✅
002-sub-features/002-at-media/016	✅	✅	✅	✅	✅	✅	✅
002-sub-features/002-at-media/017	✅	✅	✅	✅	✅	✅	✅
002-sub-features/002-at-media/018	✅	✅	✅	✅	✅	✅	✅
002-sub-features/002-at-media/at-keyframes/001	✅	✅	✅	✅	✅	✅	✅
002-sub-features/002-at-media/cycles/001	✅	❌	✅	✅	✅	✅	✅
002-sub-features/003-at-layer/001	✅	✅	✅	✅	❌	✅	✅
002-sub-features/003-at-layer/002	✅	✅	✅	✅	✅	✅	✅
002-sub-features/003-at-layer/003	✅	✅	✅	✅	❌	✅	✅
002-sub-features/003-at-layer/004	✅	✅	✅	✅	✅	✅	✅
002-sub-features/003-at-layer/005	✅	✅	✅	✅	✅	✅	✅
002-sub-features/003-at-layer/006	✅	✅	✅	✅	✅	✅	✅
002-sub-features/003-at-layer/007	✅	✅	✅	✅	✅	✅	✅
002-sub-features/003-at-layer/008	✅	✅	✅	✅	❌	✅	✅
002-sub-features/003-at-layer/009	✅	✅	✅	✅	✅	✅	✅
002-sub-features/003-at-layer/010	✅	✅	✅	✅	❌	✅	✅
002-sub-features/003-at-layer/011	✅	✅	✅	✅	✅	✅	✅
002-sub-features/003-at-layer/012	✅	✅	✅	✅	✅	✅	✅
002-sub-features/003-at-layer/013	✅	✅	✅	✅	✅	✅	✅
002-sub-features/003-at-layer/014	✅	✅	✅	❌	❌	✅	✅
002-sub-features/003-at-layer/015	✅	✅	✅	✅	❌	✅	✅
002-sub-features/003-at-layer/016	✅	✅	✅	✅	✅	✅	✅
002-sub-features/003-at-layer/017	✅	✅	✅	❌	✅	✅	✅
002-sub-features/003-at-layer/018	✅	✅	✅	❌	✅	✅	❌
002-sub-features/003-at-layer/019	✅	✅	✅	✅	❌	✅	✅
002-sub-features/003-at-layer/020	✅	✅	✅	✅	❌	✅	✅
002-sub-features/003-at-layer/at-keyframes/001	✅	✅	✅	✅	✅	✅	✅
002-sub-features/003-at-layer/at-keyframes/002	✅	✅	✅	✅	✅	✅	✅
002-sub-features/003-at-layer/case-sensitivity/001	✅	✅	✅	✅	✅	✅	✅
002-sub-features/003-at-layer/case-sensitivity/002	✅	✅	✅	✅	✅	✅	✅
002-sub-features/003-at-layer/case-sensitivity/003	✅	✅	✅	✅	✅	✅	✅
002-sub-features/003-at-layer/cycles/001	✅	❌	✅	✅	❌	✅	✅
002-sub-features/003-at-layer/mixed-importables/001	✅	✅	✅	❌	❌	✅	✅
002-sub-features/003-at-layer/url-fragments/001	✅	✅	✅	✅	❌	✅	❌
002-sub-features/004-at-supports/001	✅	✅	✅	✅	✅	✅	✅
002-sub-features/004-at-supports/002	✅	✅	✅	✅	✅	✅	✅
002-sub-features/004-at-supports/003	✅	✅	✅	✅	✅	✅	✅
002-sub-features/004-at-supports/004	✅	✅	✅	✅	✅	✅	✅
002-sub-features/004-at-supports/005	✅	✅	✅	✅	✅	✅	✅
002-sub-features/004-at-supports/006	✅	✅	✅	✅	❌	✅	✅
002-sub-features/004-at-supports/007	✅	✅	✅	✅	✅	✅	✅
002-sub-features/004-at-supports/008	✅	✅	✅	❌	✅	✅	✅
002-sub-features/004-at-supports/009	✅	✅	✅	✅	✅	✅	✅
002-sub-features/004-at-supports/010	✅	✅	✅	✅	✅	✅	✅
002-sub-features/004-at-supports/011	✅	✅	✅	✅	✅	✅	✅
002-sub-features/004-at-supports/012	✅	✅	✅	✅	✅	✅	✅
002-sub-features/004-at-supports/case-sensitivity/001	✅	✅	✅	✅	✅	✅	✅
002-sub-features/005-at-scope/001	❌	❌	❌	❌	❌	✅	❌
002-sub-features/005-at-scope/002	❌	❌	❌	❌	❌	✅	❌
002-sub-features/005-at-scope/003	❌	❌	❌	❌	❌	✅	❌
002-sub-features/005-at-scope/004	❌	❌	❌	❌	❌	✅	❌
002-sub-features/005-at-scope/005	❌	❌	❌	❌	❌	✅	❌
002-sub-features/005-at-scope/006	❌	❌	❌	❌	❌	❌	❌
002-sub-features/005-at-scope/007	❌	❌	❌	❌	❌	✅	❌
002-sub-features/005-at-scope/008	❌	❌	❌	❌	❌	✅	❌
002-sub-features/005-at-scope/009	❌	❌	❌	❌	❌	✅	❌
002-sub-features/005-at-scope/010	❌	❌	❌	❌	❌	✅	❌
002-sub-features/005-at-scope/011	❌	❌	❌	❌	❌	✅	❌
002-sub-features/005-at-scope/012	❌	❌	❌	❌	❌	✅	❌
002-sub-features/005-at-scope/case-sensitivity/001	❌	❌	❌	❌	❌	✅	❌
002-sub-features/005-at-scope/scoping/001	❌	❌	❌	❌	❌	✅	❌
002-sub-features/005-at-scope/scoping/002	❌	❌	❌	❌	❌	✅	❌
002-sub-features/005-at-scope/scoping/003	❌	❌	❌	❌	❌	✅	❌
003-should-fail/001-core-features/before-other-styles/001	✅	✅	✅	✅	❌	❌	❌
003-should-fail/001-core-features/before-other-styles/002	✅	✅	✅	✅	✅	✅	❌
003-should-fail/001-core-features/case-sensitivity/001	✅	✅	✅	❌	❌	❌	❌
004-unimplementable/001-namespace/001	✅	✅	✅	❌	❌	❌	❌
004-unimplementable/002-url-queries/001	✅	✅	✅	❌	❌	✅	❌
004-unimplementable/004-subresource/001	✅	✅	✅	❌	❌	❌	❌
004-unimplementable/004-subresource/002	✅	✅	✅	❌	❌	❌	❌
005-implementation-specific/leading-slash-is-import-root/001	✅	✅	✅	❌	❌	❌	✅
005-implementation-specific/leading-slash-is-import-root/002	✅	✅	✅	❌	❌	❌	❌
999-irrelevant/url-format/001/absolute-url	✅	✅	✅	✅	✅	✅	✅
999-irrelevant/url-format/001/default	✅	✅	✅	✅	✅	✅	✅
999-irrelevant/url-format/001/relative-url	✅	✅	✅	✅	✅	✅	✅
999-irrelevant/url-fragments/001	✅	✅	✅	❌	❌	✅	❌
Total	134 / 151	130 / 151	135 / 151	118 / 151	94 / 151	150 / 151	112 / 151

Contributing

Any test contributions are welcome!

You can open an issue and describe the case you want to test, or you can open a PR with a new test.

I don't care about technical purity, formatting, duplicates, ...
Anything that increases test coverage is very much appreciated.