Improve compatibility with Safari 15

[philipp-spiess]

This PR improves the compatibility with Tailwind CSS v4 with unsupported browsers with the goal to greatly improve compatibility with Safari 15.

To make this work, this PR makes the following changes to all code

• Change oklab(…) default theme values to use a percentage in the first place (so instead of --color-red-500: oklch(0.637 0.237 25.331); we now define it as --color-red-500: oklch(63.7% 0.237 25.331); since this syntax has much broader support on Safari).
• Polyfill @property with a @supports query targeting older versions of Safari and Firefox *
• Create fallbacks for the color-mix(…) function that use inlined color values from your theme so that they can be computed a compile time by lightningcss. These fallbacks will convert to srgb to increase compatibility.
• Create fallbacks for the relative color feature used in the new shadow utilities and using color-mix(…) in case relative color is applied on currentcolor (due to limited browser support)
• Create fallbacks for gradient interpolation methods (e.g. to support bg-linear-to-r/oklab)
• Polyfill @media queries range syntax.

A simplified example

Given this example CSS input:

@import 'tailwindcss';
@source inline('from-cyan-500/50 bg-linear-45');

Here's the updated output CSS including the newly added polyfills and updated oklab values:

.bg-linear-45 {
  --tw-gradient-position: 45deg;
  background-image: linear-gradient(var(--tw-gradient-stops));
}

@supports (background-image: linear-gradient(in lab, red, red)) {
  .bg-linear-45 {
    --tw-gradient-position: 45deg in oklab;
  }
}

.from-cyan-500\\/50 {
  --tw-gradient-from: oklab(71.5% -.11682 -.08247 / .5);
  --tw-gradient-stops: var(--tw-gradient-via-stops, var(--tw-gradient-position), var(--tw-gradient-from) var(--tw-gradient-from-position), var(--tw-gradient-to) var(--tw-gradient-to-position));
}

@supports (color: color-mix(in lab, red, red)) {
  .from-cyan-500\\/50 {
    --tw-gradient-from: color-mix(in oklab, var(--color-cyan-500) 50%, transparent);
  }
}

:root, :host {
  --color-cyan-500: oklch(71.5% .143 215.221);
}

@supports (((-webkit-hyphens: none)) and (not (margin-trim: 1lh))) or ((-moz-orient: inline) and (not (color: rgb(from red r g b)))) {
  @layer base {
    *, :before, :after, ::backdrop {
      --tw-gradient-position: initial;
      --tw-gradient-from: #0000;
      --tw-gradient-via: #0000;
      --tw-gradient-to: #0000;
      --tw-gradient-stops: initial;
      --tw-gradient-via-stops: initial;
      --tw-gradient-from-position: 0%;
      --tw-gradient-via-position: 50%;
      --tw-gradient-to-position: 100%;
    }
  }
}

@property --tw-gradient-position {
  syntax: "*";
  inherits: false
}

@property --tw-gradient-from {
  syntax: "<color>";
  inherits: false;
  initial-value: #0000;
}

@property --tw-gradient-via {
  syntax: "<color>";
  inherits: false;
  initial-value: #0000;
}

@property --tw-gradient-to {
  syntax: "<color>";
  inherits: false;
  initial-value: #0000;
}

@property --tw-gradient-stops {
  syntax: "*";
  inherits: false
}

@property --tw-gradient-via-stops {
  syntax: "*";
  inherits: false
}

@property --tw-gradient-from-position {
  syntax: "<length-percentage>";
  inherits: false;
  initial-value: 0%;
}

@property --tw-gradient-via-position {
  syntax: "<length-percentage>";
  inherits: false;
  initial-value: 50%;
}

@property --tw-gradient-to-position {
  syntax: "<length-percentage>";
  inherits: false;
  initial-value: 100%;
}

* A note on @property polyfills and CSS modules

On Next.js, CSS module files are required to be pure, meaning that all selectors must either be scoped to a class or an ID. Fortunatnyl for us, this does not apply to @property rules which we've been using before to initialize CSS variables.

However, since we're now bringing back the @property polyfills, that would cause unexpected rules to be exported from the CSS file as this:

@reference "tailwindcss";

.skew {
  @apply skew-7;
}

Would turn to the following file:

.skew {
  /* … */
}
@supports (/*…*/) {
  @layer base {
    *, :before, :after, ::backdrop {
      --tw-gradient-position: initial;
    }
  }
}
@property /* … */ 

Notice that this adds a * selector which is not considered pure.

Unfortunately there is no way for us to silence this warning or work around it, as the dependency causing this errors (postcss-modules-local-by-default) is bundled into Next.js. To work around crashes, these polyfills will not apply to CSS modules processed by the PostCSS extension for now.

Testing on tailwindcss.com

To see the changes in effect, take a look at this screencast that compares tailwindcss.com on iOS 15.5 with a version that has the patches of this PR applied:

Screen.Recording.2025-03-28.at.17.52.25.mov

Open questions

• Should we make the @property polyfill not require an @layer base and instead place the generated CSS on the top of the file?
• Add fallbacks for newly introduced usages of the relative color syntax.
• Decide if we're ok with how we actually implement the @media range query workaround

Test plan

• Tested on tailwindcss.com via a preview build: https://tailwindcss-com-git-legacy-browsers-tailwindlabs.vercel.app/
• Updated tests
• Ensure we also test on Chrome 111, Safari 16.4, Firefox 128 to make sure we have no regressions. Also tested on Safari 16.4, 15.5, 18.0

[RobinMalfait]

Haven't functionally tested yet, but wanted to leave these notes already.

[RobinMalfait]

😅

[RobinMalfait]

Sad that this means that you can't change the --my-half value at runtime, but if it's not supported what can you do haha.

[RobinMalfait]

I wonder if we should store the polyfills on the designSystem itself 🤔

[silvenon]

Sorry for the late comment, but it is my understanding that with this PR merged the general Safari support for basic Tailwind v4 goes from 16.4 to 16.2 (color-mix is supported down to 16.2). Is that true?