vite-plugin-svelte chokes on custom media syntax when using vitePreprocess and Lightning CSS

[knpwrs]

Describe the bug

I just spun up a new skeleton sveltekit project which by default uses vitePreprocess. I installed lightning css (npm i -D lightningcss) I have modified vite.config.ts to contain the following:

import { sveltekit } from '@sveltejs/kit/vite';
import { defineConfig } from 'vite';

export default defineConfig({
  plugins: [sveltekit()],
  css: {
    transformer: 'lightningcss',
    lightningcss: {
      drafts: {
        nesting: true,
        customMedia: true,
      },
    },
  },
});

Given the following +page.svelte file, I can see the nesting configuration works:

<h1>Welcome to SvelteKit</h1>

<style>
  /* @custom-media --small-viewport (max-width: 30em); */

  h1 {
    color: blue;

    &:hover {
      color: red;
    }

    /* @media (--small-viewport) { */
    /*   color: green; */
    /* } */
  }

</style>

With the configuration set as in my first code block, hovering the <h1> turns it red. If I comment out the nesting configuration, hovering the <h1> does nothing.

However, if I uncomment the custom media, I get a syntax error:

Internal server error: /Users/knpwrs/Downloads/my-app/src/routes/+page.svelte:4:43 ")" is expected
  Plugin: vite-plugin-svelte
  File: .../my-app/src/routes/+page.svelte:4:43
   2 |
   3 |  <style>
   4 |    @custom-media --small-viewport (max-width: 30em);
                                                   ^

Reproduction URL

https://github.com/knpwrs/sveltekit-lightningcss-custom-media-issue

Reproduction

1. npm create svelte@latest my-app
2. cd my-app
3. npm install -D lightningcss
4. Update configuration as shown above
5. Observe bug

Logs

logs.txt

System Info

System:
    OS: macOS 13.4
    CPU: (12) arm64 Apple M2 Max
    Memory: 1.26 GB / 64.00 GB
    Shell: 5.9 - /opt/homebrew/bin/zsh
  Binaries:
    Node: 20.3.1 - /opt/homebrew/bin/node
    Yarn: 1.22.19 - /opt/homebrew/bin/yarn
    npm: 9.6.7 - /opt/homebrew/bin/npm
  Browsers:
    Chrome: 116.0.5845.140
    Chrome Canary: 117.0.5863.0
    Safari: 16.5
  npmPackages:
    @sveltejs/adapter-auto: ^2.0.0 => 2.1.0
    @sveltejs/kit: ^1.20.4 => 1.24.0
    svelte: ^4.0.5 => 4.2.0
    vite: ^4.4.2 => 4.4.9

[dominikg]

The issue is caused here: https://github.com/sveltejs/vite-plugin-svelte/blob/50ce3165f613a15f0ee12c1e68c064e0fc0d0f1f/packages/vite-plugin-svelte/src/preprocess.js#L67

your style block does not have a "lang" attribute and then vitePreprocess skips it entirely. You can work around it with the current version of vite-plugin-svelte by setting lang="css".

@bluwy any reason not to fall back to 'css' if lang isn't set?

[knpwrs]

That works! It's even possible to put custom media queries in a separate file and import it e.g., @import 'media.css'; or @import '$lib/media.css';.

Thank you! Looking forward to seeing how this is resolved long term.

[bluwy]

I think it was to keep it compatible with svelte-preprocess before, which only preprocesses things with the lang attribute. The issue is similar to postcss not applying its transform because you don't have lang="postcss" set.

Technically Vite always run postcss or lightningcss on all CSS files though, so maybe in the next v-p-s major we can change to always run the preprocessing.

[dominikg]

great point about this being a breaking change and also mentioning lang="postcss".

Makes sense to have vitePreprocess process all style nodes with supported langs, but this is going to make it more different from svelte-preprocess. I wonder if we need to document those differences to avoid confusion for users.

cc @kaisermann

[knpwrs]

I'm not sure what the interaction between svelte-check and vite-plugin-svelte is, but lang="css" appears to cause linting issues with custom media:

❯ npm run check

> web-svelte@0.0.1 check
> svelte-kit sync && svelte-check --tsconfig ./tsconfig.json

====================================
Loading svelte-check in workspace: /Users/knpwrs/Workspace/letschurch/lets.church/apps/web-svelte
Getting Svelte diagnostics...

/Users/knpwrs/Workspace/letschurch/lets.church/apps/web-svelte/src/routes/+page.svelte:5:3
Warn: Unknown at rule @custom-media (css)
<style lang="css">
  @custom-media --small-viewport (max-width: 30em);

====================================
svelte-check found 0 errors and 1 warning in 1 file

If I set lang="postcss" that warning does not appear (even though I don't have postcss configured at all, nevermind a custom media plugin for postcss):

❯ npm run check

> web-svelte@0.0.1 check
> svelte-kit sync && svelte-check --tsconfig ./tsconfig.json

====================================
Loading svelte-check in workspace: /Users/knpwrs/Workspace/letschurch/lets.church/apps/web-svelte
Getting Svelte diagnostics...

====================================
svelte-check found 0 errors and 0 warnings

Would this warrant a new issue in this repo or another? Or would a fix here also fix the svelte-check issue?

This behavior reproduces with the LSP as well:

[dominikg]

good question. i assume it still preprocesses the style block with lightningcss if you set lang="postcss" in your code? if yes that would be the workaround for that one.

Maybe we should add lang="lightningcss" too ?

cc @dummdidumm

[knpwrs]

I hadn't even thought to try that, but yes indeed, that does work and lightningcss still processes the style block.

[kaisermann]

I think it was to keep it compatible with svelte-preprocess before, which only preprocesses things with the lang attribute. The issue is similar to postcss not applying its transform because you don't have lang="postcss" set.

@bluwy in theory, if postcss is configured, svelte-preprocess will run postcss on style blocks without the lang attribute.