Make postcss a peer dependency

[Demivan]

Why is postcss a regular dependency and not a peer-dependency like other CSS processors?

[solarkraft]

I assume this is to keep everything simple. As a user of Vite I don't even have to know what postcss is until I want to use its features. I generally like this approach.

However of course when I do want to do this things start to become hairy: The plugin postcss-nesting has postcss as a peer dependency, meaning that the package manager will throw a warning or, in pnpm's case, an error by default (I assume they don't look into the installed packages' dependencies to avoid phantom dependencies). This (please correct me if I'm wrong, I very well could be since I just started learning about this) kind of sucks, since the version I declare as a dependency (simply to satisfy the peer dependency, never actually using it since everything happens inside of Vite) could diverge from the version Vite actually uses. This would only be noticeable once something breaks if we don't manually keep Vite's version of postcss in sync with our version of postcss (postcss-nesting will see that our project has a postcss version as a dependency that it'll work with, but the version Vite uses could be a completely different one!).

This would indeed not be an issue if Vite had PostCSS as a peer dependency, but then the user would have to install it manually

Keeping things simple is a worthy goal, which could make this a reasonable trade-off. Then again most people use templates rather than installing Vite manually.