CSS output does not respect Vite build target

[hyunbinseo]

Describe the bug

Browser native CSS nesting should be flattened based on the Vite default build.target, but is not.

Reproduction

Vanilla Vite project correctly builds the following CSS:

import './style.css';

document.querySelector<HTMLDivElement>('#app')!.innerHTML = `<span>Hello,&nbsp;</span>`;

/* Nesting requires Chrome 120, Safari 17.2 */
/* Reference https://caniuse.com/css-nesting */
span {
  color: red;
  &::after {
    content: 'World!';
  }
}

/* Default option (targeting Safari 14)  */
span{color:red}span:after{content:"World!"}

/* With build: { cssTarget: 'safari17.2' } */
span{color:red;&:after{content:"World!"}}

---

This is not the case with a SvelteKit template:

┌  Welcome to the Svelte CLI! (v0.8.1)
│
◇  Which template would you like?
│  SvelteKit minimal
│
◇  Add type checking with TypeScript?
│  Yes, using TypeScript syntax
│
◆  Project created
│
◇  What would you like to add to your project? (use arrow keys / space bar)
│  none

<!-- src\routes\+page.svelte -->
<span>Hello,&nbsp;</span>

<style>
  span {
    color: red;
    &::after {
      content: 'World!';
    }
  }
</style>

/* .svelte-kit\output\client\_app\immutable\assets\_page.CmDdIB0i.css */
/* With build: { cssTarget: 'safari14' } */
span.svelte-yc2s4t{color:red}span.svelte-yc2s4t:after{content:"World!"}

/* .svelte-kit\output\client\_app\immutable\assets\_page.D4Ercq9S.css */
/* Default option (targeting Safari 14)  */
span.svelte-yc2s4t{color:red;&:after{content:"World!"}}

Logs

System Info

System:
  OS: Windows 11 10.0.26100
  CPU: (8) x64 Intel(R) Core(TM) Ultra 7 258V
  Memory: 9.22 GB / 31.51 GB
Binaries:
  Node: 22.14.0 - ~\AppData\Local\fnm_multishells\12668_1745374883481\node.EXE
  npm: 10.9.2 - ~\AppData\Local\fnm_multishells\12668_1745374883481\npm.CMD
  pnpm: 10.9.0 - ~\AppData\Local\fnm_multishells\12668_1745374883481\pnpm.CMD
Browsers:
  Edge: Chromium (131.0.2903.112)
npmPackages:
  @sveltejs/adapter-auto: ^4.0.0 => 4.0.0 
  @sveltejs/kit: ^2.16.0 => 2.20.7 
  @sveltejs/vite-plugin-svelte: ^5.0.0 => 5.0.3 
  svelte: ^5.0.0 => 5.28.2 
  vite: ^6.2.5 => 6.3.2

Severity

serious, but I can work around it

Additional Information

No response