Non-modern build should also ("pre")load CSS

Description

While I’m doing my efforts in sveltejs/kit#6265 to add legacy support to SvelteKit on the Vite way, I had noticed that when navigating from one page to the other, the internal function __vitePreload is called, which is defined in the plugin importAnalysisBuild.

As far as I understand, the purpose of this function is to preload JS modules and also to (pre?)load CSS files. On legacy output chunks, it shuts down the preloading ability. I understand that there is no need to preload JS modules on non-modern(i.e. non ESModule compatible) browsers, but for the correct way of loading CSS, the CSS files should(?) be loaded in __vitePreload.

According to my local tests, if the CSS are getting preloaded on legacy builds, everything works great in my legacy work on SvelteKit.

Suggested solution

Change the code of the plugin importAnalysisBuild to (pre?)load CSS anyway, and when the browser is also modern, import also the preloaded JS files.

Alternative

You tell me?

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn’t already an issue that request the same feature to avoid creating a duplicate.

[sapphi-red]

It’s described here. https://github.com/vitejs/vite/issues/2062#issuecomment-782385406

[Tal500]

Hello Tal500. I appreciate your effort put in here. I was trying out your solution on our code, but the production build was failing due to this: Is this issue in any way related? [vite-plugin-svelte-kit] Prerendering failed with code 1 error during build: Error: Prerendering failed with code 1 at ChildProcess.<anonymous> (file:///Users/fejirogospel/Documents/GitHub/center-nx/apps/site-app/node_modules/@sveltejs/kit/src/exports/vite/index.js:455:14) at ChildProcess.emit (node:events:527:28) at Process.ChildProcess._handle.onexit (node:internal/child_process:291:12)

Hello! Are you talking about the SvelteKit legacy PR I made? If so, you better comment this issue on the PR I sent on SvelteKit, not here.(was it a mistake?)

Anyway, for your case, can you:

1. Disable prerendering (in svelte.config.js), do npm run build && npm run preview, and share the result? Also, can you please publish a bigger chunk of the console error? It’s seems that a detailed error message appears earlier.
2. Can you give a sample code for reproduction?
3. Did you try to compile and run the simple demo?

Please comment to me to this on the PR in SvelteKit, for a better discussion.