CSS output does not respect Vite build target

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

Reproduce the issue from src/routes/+page.svelte in a SvelteKit minimal project, comparing the default build target with an explicit cssTarget. Trace how the generated CSS output is produced and verify that the default target flattens the nested selector while the newer target preserves it.