Tailwind @apply breaks with Svelte 5 (alpha 57)

[svelte-kit-so-good]

Describe the bug

Video shows it best; starting with alpha 57 the tailwind @apply rule breaks (specifically with darkmode):

tailwind-break-alpha-57.mp4

Reproduction

SvelteLab

Logs

No response

System Info

Svelte v5.0.0-next.57 (breaks)
Svelte v5.0.0-next.56 (works)

Severity

annoyance

[mwc]

@svelte-kit-so-good

1. Update Tailwindcss to ^v3.4.1
2. Modify the darkMode configuration in tailwind.config.mjs:

darkMode: ['variant', '&:where(:global(.dark), :global(.dark) *)']

3. Configure darkMode for Astro + Svelte
   If you are using Astro + Svelte, you need to configure darkMode differently in your Tailwind config file:

darkMode: ['variant', ['&:where(.dark, .dark *)', '&:where(:global(.dark), :global(.dark) *)']]

I'm not sure if this is the best practice, but it works for the current changes.

[dummdidumm]

The generated CSS is this:

    main.svelte-11sclw {
          /* tailwind @apply with 'dark:' breaks in alpha 57 */
      --tw-bg-opacity:1;
      background-color:rgb(219 234 254 / var(--tw-bg-opacity));
    }
    /* (unused) :is(html.dark-mode main) {
          --tw-bg-opacity:1;
      background-color:rgb(30 58 138 / var(--tw-bg-opacity));
}*/
    button.svelte-11sclw {
          
      width:50px;
      aspect-ratio:1/1;
      border-radius:10px;
      background:black;
      /* regular css works */
    }
    html.dark-mode button.svelte-11sclw {
                filter:invert(1);
      }

The problem is that Tailwind generates CSS which contains selectors that look unused to Svelte. The solution is to add adjust the Tailwind config by adding :global like this:

/** @type {import('tailwindcss').Config} */
export default {
-  darkMode: ["class", "html.dark-mode"],
+  darkMode: ["class", ":global(html.dark-mode)"],
  content: ["./src/**/*.{html,js,svelte,ts}"],
  theme: {
    extend: {},
  },
  plugins: [],
};

Question: Was this an existing config from a Svelte 4 project? If so, we may need to adjust the breaking changes section to note that Svelte now supports :is and :where properly and therefore also adds the CSS treeshaking to it.

[svelte-kit-so-good]

Question: Was this an existing config from a Svelte 4 project? If so, we may need to adjust the breaking changes section to note that Svelte now supports :is and :where properly and therefore also adds the CSS treeshaking to it.

Yes. A config that worked from Svelte 4 up to and including Svelte 5 Alpha 56.

Edit:
Oh .. yea no this is a non-solution. So my issue was only with using the @apply rule for the tailwind 'dark:' selector inside the <style> section of the svelte component. What you're suggesting now breaks more of the 'dark:' tailwind functionality ...specifically all inline use cases are broken now too, e.g.:

<div class="dark:bg-blue-900"/> 
<!--  even with breaking changes in alpha 57, this still worked-->
<!--  Breaks with your tailwind config :global() suggestion -->

[dummdidumm]

Reopening and giving it the documentation level - we should note this in the breaking changes.

[AdrianGonz97]

The problem is that Tailwind generates CSS which contains selectors that look unused to Svelte. The solution is to add adjust the Tailwind config by adding :global like this:

/** @type {import('tailwindcss').Config} */
export default {
-  darkMode: ["class", "html.dark-mode"],
+  darkMode: ["class", ":global(html.dark-mode)"],
  content: ["./src/**/*.{html,js,svelte,ts}"],
  theme: {
    extend: {},
  },
  plugins: [],
};

Unfortunately, this isn't a fix. While it does fix the cases where @apply is used, everywhere else breaks now. So the problem is now the inverse, where styles defined inline with the dark: modifier (e.g. class="dark:bg-blue-900") are now the broken case.

Edit: Here's a REPL that I find to be a just bit easier to play with and demonstrate.

[svelte-kit-so-good]

With great pain comes great responsibility ...

TL;DR: Uno continues working; Tailwind breaks from Svelte 5 Alpha 57

I went ahead and made a Stackblitz for UnoCSS and Tailwind. In the <style> section of the svelte component the:

• @apply rule with 'dark:' breaks with both Tailwind and Uno (using Tailwind plugin for Uno)
• --at-apply rule of Uno with 'dark:' works!

[mwc]

REPL, after modifying tailwind.config.cjs as shown below, your code doesn't work? @svelte-kit-so-good

/** @type {import('tailwindcss').Config}*/
const config = {
- darkMode: "class",
+ darkMode: ['variant', ['&:where(.dark, .dark *)', '&:where(:global(.dark), :global(.dark) *)']],

  content: ["./src/**/*.{html,js,svelte,ts}"],

  theme: {
    extend: {},
  },

  plugins: [],
};

module.exports = config;

[svelte-kit-so-good]

@mwc yes this is a solution, but I would venture to say it's not a satisfactory one since:

1. it breaks the default behaviour with Tailwind, that just worked' up until alpha 57 — even before Svelte 5.
2. As I showed with Uno stackblitz, the default behaviour continues to work

Personally with Tailwind I have just left things inline or have moved things to the style section like:

<style lang="postcss">
	.some-class {
		@apply some-utility-classes;
		:global(html.dark) & {
			@apply some-more utility-classes;
		}
	}
</style>