`vite build` produces incorrectly flattened nested CSS

[RobinMalfait]

Describe the bug

When using modern CSS with nesting, a production build will generate invalid CSS.

Given the following CSS:

.foo {
  :where(& > .bar) {
    color: red;
  }
}

In a production build, this is flattened as:

.foo :where(>.bar){color:red}

This CSS is invalid because :where(…) doesn't accept relative selectors.

But I expect it to be flattened as this instead:

:where(.foo>.bar){color:red}

---

Note: in dev mode (vite dev) it works as expected.

Reproduction

https://github.com/RobinMalfait/vite-incorrect-css-flattening

Steps to reproduce

• npm install
• npm run dev — open http://localhost:5173, the text should be red
• npm run build — open http://localhost:4173, the text should be red, but is black

System Info

System:
    OS: macOS 15.2
    CPU: (10) arm64 Apple M1 Max
    Memory: 3.51 GB / 64.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 18.17.1 - ~/.local/state/fnm_multishells/16285_1734176270818/bin/node
    Yarn: 1.22.22 - ~/.local/state/fnm_multishells/1076_1734173982354/bin/yarn
    npm: 9.6.7 - ~/.local/state/fnm_multishells/16285_1734176270818/bin/npm
    pnpm: 9.8.0 - ~/Library/pnpm/pnpm
    bun: 1.1.39 - ~/.bun/bin/bun
  Browsers:
    Brave Browser: 120.1.61.114
    Chrome: 131.0.6778.140
    Chrome Canary: 133.0.6898.0
    Edge: 124.0.2478.80
    Firefox Nightly: 131.0a1
    Safari: 18.2
    Safari Technology Preview: 18.2
  npmPackages:
    vite: ^6.0.3 => 6.0.3

Used Package Manager

npm

Logs

Click to expand!

❯ vite build --debug
  vite:config no config file found. +0ms
  vite:config using resolved config: {
  vite:config   root: '/Users/robin/github.com/RobinMalfait/vite-incorrect-css-flattening',
  vite:config   base: '/',
  vite:config   mode: 'production',
  vite:config   configFile: undefined,
  vite:config   logLevel: undefined,
  vite:config   clearScreen: undefined,
  vite:config   build: {
  vite:config     target: [ 'es2020', 'edge88', 'firefox78', 'chrome87', 'safari14' ],
  vite:config     polyfillModulePreload: true,
  vite:config     modulePreload: { polyfill: true },
  vite:config     outDir: 'dist',
  vite:config     assetsDir: 'assets',
  vite:config     assetsInlineLimit: 4096,
  vite:config     sourcemap: false,
  vite:config     terserOptions: {},
  vite:config     rollupOptions: {},
  vite:config     commonjsOptions: { include: [ /node_modules/ ], extensions: [ '.js', '.cjs' ] },
  vite:config     dynamicImportVarsOptions: { warnOnError: true, exclude: [ /node_modules/ ] },
  vite:config     write: true,
  vite:config     emptyOutDir: null,
  vite:config     copyPublicDir: true,
  vite:config     manifest: false,
  vite:config     lib: false,
  vite:config     ssrManifest: false,
  vite:config     ssrEmitAssets: false,
  vite:config     reportCompressedSize: true,
  vite:config     chunkSizeWarningLimit: 500,
  vite:config     watch: null,
  vite:config     cssCodeSplit: true,
  vite:config     minify: 'esbuild',
  vite:config     ssr: false,
  vite:config     emitAssets: true,
  vite:config     createEnvironment: [Function: createEnvironment],
  vite:config     cssTarget: [ 'es2020', 'edge88', 'firefox78', 'chrome87', 'safari14' ],
  vite:config     cssMinify: true
  vite:config   },
  vite:config   environments: {
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  vite:config       define: undefined,
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  vite:config   publicDir: '/Users/robin/github.com/RobinMalfait/vite-incorrect-css-flattening/public',
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  vite:config   command: 'build',
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  vite:config   mainConfig: null,
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  vite:config   json: { namedExports: true, stringify: 'auto' },
  vite:config   esbuild: { jsxDev: false },
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  vite:config   packageCache: Map(1) {
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  vite:config       data: {
  vite:config         name: 'vite-incorrect-css-flattening',
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  vite:config         description: '',
  vite:config         main: 'index.js',
  vite:config         scripts: { dev: 'vite dev', build: 'vite build && vite preview' },
  vite:config         keywords: [],
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  vite:config   },
  vite:config   optimizeDeps: {
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  vite:config     needsInterop: [],
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  vite:config   dev: {
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  vite:config   fsDenyGlob: [Function: arrayMatcher],
  vite:config   safeModulePaths: Set(0) {}
  vite:config } +13ms
vite v6.0.3 building for production...
✓ 2 modules transformed.
dist/index.html                 0.20 kB │ gzip: 0.15 kB
dist/assets/index-DiaIqj75.css  0.03 kB │ gzip: 0.05 kB
✓ built in 41ms

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