What version of Tailwind CSS are you using?

v4.3.1

I reproduced this with the standalone CLI distributed through tailwindcss-ruby 4.3.1. This project also has tailwindcss@4.3.1 installed via npm.

What build tool are you using?

Tailwind CLI directly:

tailwindcss -i input.css -o output.css --minify

The same warning also appears with --optimize.

What version of Node.js are you using?

Not using a Node-based build in the reproduction. This came from a Rails app using tailwindcss-rails 4.6.0 / tailwindcss-ruby 4.3.1.

What browser are you using?

N/A, build-time warning.

What operating system are you using?

macOS arm64.

Reproduction URL

No repo needed; this reproduces with a single input file:

@import "tailwindcss";

.foo {
  position-try-fallbacks: --flip-above;
}

@position-try --flip-above {
  top: auto;
  bottom: 0;
}

Run:

tailwindcss -i input.css -o output.css --minify

Describe your issue

Tailwind's optimizer emits a warning for @position-try:

Found 1 warning while optimizing generated CSS:

│   position-try-fallbacks: --flip-above;
│ }
│ @position-try --flip-above {
┆              ^-- Unknown at rule: @position-try
┆
│   top: auto;
│   bottom: 0;

Done in 102ms

The command exits successfully and the at-rule is preserved in the output CSS, but the warning is noisy for valid CSS Anchor Positioning syntax.

This shows up in a real app when importing intl-tel-input/styles from intl-tel-input@29.1.1, which includes this rule for a detached country selector fallback:

.iti--detached-country-selector {
  top: calc(anchor(bottom) + 3px);
  left: anchor(left);
  position-try-fallbacks: --iti-flip-above;
}
@position-try --iti-flip-above {
  top: auto;
  bottom: calc(anchor(top) + 3px);
}

Expected behavior: Tailwind's optimization/minification should not warn on @position-try, or should otherwise expose a way to avoid warning on newer standards-based at-rules that are intentionally passed through.

If this belongs upstream in Lightning CSS rather than Tailwind, guidance would be appreciated.