fix: remove unused SSR CSS files

[eltigerchino]

fixes #9161

This PR prevents copying the unused(?) CSS files that are generated from vite's build.ssrEmitAssets option. Kit seemingly only uses the client optimised CSS files in the .svelte-kit client bundle, so these server bundle css files shouldn't be copied over.

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[benmccann]

This doesn't seem safe to me. I'm not sure that we can know that the CSS is unused on the server. I think if it gets imported from a server file then this will break (#5240).

I had an earlier PR to fix the paths when you don't set paths.relative: true and I believe it got reverted. We could try to get that PR back in without whatever issue it was causing, which would solve this in most cases by avoiding the creation of duplicate CSS files (as long as you don't set paths.relative - maybe we could also fix that one by using relative paths on the server, but I'm not sure)

[eltigerchino]

This doesn't seem safe to me. I'm not sure that we can know that the CSS is unused on the server. I think if it gets imported from a server file then this will break (#5240).

I don't think they're used anywhere because:

1. import './styles.css' the server asset is never referenced in the built output.
2. import style from './styles.css?inline' is always stored as a variable in the built js file.

I had an earlier PR to fix the paths when you don't set paths.relative: true and I believe it got reverted. We could try to get that PR back in without whatever issue it was causing, which would solve this in most cases by avoiding the creation of duplicate CSS files (as long as you don't set paths.relative - maybe we could also fix that one by using relative paths on the server, but I'm not sure)

The problem now also lies in the difference of filenames as well. I'm not sure what changed but the client css assets are now always named after the nodes such as 0.abs34af.css instead of _page.sd12sd.css. Building with the latest version of kit, there will always be duplicate CSS files even without the URL(...) path differences.

[benmccann]

The case I was thinking of would be if you had a +server.js file with something like:

import css from './example.css';

export function GET() {
    return new Response(css + '\n\np { color: red }');
}

I don't know how likely that is, but it'd also be pretty weird that you can import anything you want into a +server.js file except for .css files which would magically break.

[eltigerchino]

import css from './example.css';

I’ve tested this and the CSS gets inlined as well. Furthermore, this syntax is deprecated by Vite and is equivalent to import css from './example.css?inline'. Although, I’m not sure what that would mean once Vite officially stops supporting it.

[benmccann]

It looks like Svelte 5 will stop outputting CSS in SSR mode. I wonder if we should just wait until then to avoid the extra code complication here

[eltigerchino]

I agree. I think it's alright to wait until Svelte 5 releases.