_N_E_STYLE_LOAD insert function in CSS loader is not transpiled, breaks on old browsers

[FeNoMeNa]

To Reproduce

1. Create a Next.js app with any CSS imports
2. Set a legacy browser target (e.g. chrome >= 45)
3. Run next build --webpack
4. Open the app in Chrome 45

Current vs. Expected behavior

Expected Behavior
The insert function should be transpiled to ES5, or at minimum respect the project's browserslist/output.environment webpack configuration.

Actual Behavior
The webpack runtime chunk contains untranspiled modern syntax injected directly as a serialized string from packages/next/src/build/webpack/config/blocks/css/index.ts:

(function(linkTag) {
  if (typeof _N_E_STYLE_LOAD === 'function') {
    const { href, onload, onerror } = linkTag;  // ← not transpiled
    ...
  }
})(linkTag)

This crashes on old browsers with Unexpected token {.

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #1 SMP PREEMPT_DYNAMIC Thu Jun  5 18:30:46 UTC 2025
  Available memory (MB): 31815
  Available CPU cores: 22
Binaries:
  Node: 22.22.0
  npm: 10.9.4
  Yarn: 1.22.22
  pnpm: N/A
Relevant Packages:
  next: 16.1.6 // Latest available version is detected (16.1.6).
  eslint-config-next: N/A
  react: 19.2.3
  react-dom: 19.2.3
  typescript: N/A
Next.js Config:
  output: export
Done in 0.84s.

Which area(s) are affected? (Select all that apply)

CSS

Which stage(s) are affected? (Select all that apply)

next build (local)

Additional context

No response