[plugin-legacy] Legacy build for IE 11 contains hexadecimal RGBA notation

[winterkind]

Describe the bug

In my legacy build for IE 11, the built CSS chunk contains hexadecimal RGBA notation, which IE 11 does not support. Unlike the JS files, no separate legacy chunk for CSS is built.

From what I read so far, since version 2.5 vite uses esbuild to minify CSS files. Through issue #4650 there was an enhancement to respect the given target configuration in the main build. However, nothing like that seems to exist for the configuration of the legacy build. Apologies if I am missing something obvious.

As a result, the styling in broken in IE 11.

Reproduction

1. Clone from https://gitlab.com/winterkind/vite-svelte-ie11
2. npm install
3. npm run build
4. Check the built CSS chunk in the dist folder. You will find hexadecimal RGBA notation, e.g. for background-color. In the source file this is in functional notation.

System Info

System:
    OS: Windows 10 10.0.19042
    CPU: (8) x64 Intel(R) Core(TM) i7-7820HQ CPU @ 2.90GHz
    Memory: 16.22 GB / 31.87 GB
  Binaries:
    Node: 14.17.4 - ~\AppData\Local\Volta\tools\image\node\14.17.4\node.EXE
    Yarn: 1.22.11 - ~\AppData\Local\Volta\tools\image\yarn\1.22.11\bin\yarn.CMD
    npm: 6.14.14 - ~\AppData\Local\Volta\tools\image\npm\6.14.14\bin\npm.CMD
  Browsers:
    Chrome: 93.0.4577.63
    Edge: Spartan (44.19041.1023.0), Chromium (93.0.961.47)
    Internet Explorer: 11.0.19041.906
  npmPackages:
    vite: ^2.5.7 => 2.5.7

Used Package Manager

npm

Logs

No response

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