Don't use color-mix(…) on currentColor (tailwindlabs#17247)

Closes tailwindlabs#17194

This PR works around a crash when rendering opacity on `currentColor`
(as used by the placeholder styles in preflight) on Safari 16.4 and
Safari 16.5. Unfortunately it seems that the [`color-mix(…)` function is
not compatible with `currentColor` for these versions of
Safari](https://stackoverflow.com/questions/76436497/the-color-mix-property-involving-currentcolor-causes-safari-to-crash).
We tried a few different ways to work around this without success:

- Using an `@supports` media query to target these Safari versions and
overwriting the placeholder still makes these browsers crash.
- Changing the way we apply opacity to `currentColor` in core doesn't
seem to work for non-placeholder values:
tailwindlabs#17194 (comment)
However, a wrong opacity is still better than a complete browser crash.

The work-around of using the `oklab(…)` function does seem to work for
`::placeholder` styles in preflight though according to our testing so
this PR applies this change to preflight.

## Test plan

- See https://play.tailwindcss.com/WSsSTLHu8h?file=css
- Tested on Chrome/Safari 16.4/Safari 18.3/Firefox

<img width="564" alt="Screenshot 2025-03-17 at 11 32 47"
src="https://github.com/user-attachments/assets/cfd0db71-f39a-4bc0-bade-cea70afe50ae"
/>

Author: philipp-spiess

packages/tailwindcss/preflight.css

@@ -285,12 +285,12 @@ textarea,
 
 /*
   1. Reset the default placeholder opacity in Firefox. (https://github.com/tailwindlabs/tailwindcss/issues/3300)
-  2. Set the default placeholder color to a semi-transparent version of the current text color.
+  2. Set the default placeholder color to a semi-transparent version of the current text color. We use the `oklab(…)` function to work around an issue in Safari 16.4 and 16.5. (https://github.com/tailwindlabs/tailwindcss/issues/17194)
 */
 
 ::placeholder {
   opacity: 1; /* 1 */
-  color: color-mix(in oklab, currentColor 50%, transparent); /* 2 */
+  color: oklab(from currentColor l a b / 50%); /* 2 */
 }
 
 /*