Following up with my comment...
$ lightningcss --version
1.0.0-alpha.34
What I would have expected is it to match the tag (with a small preference for without the v* but easy to manage with it if it's easier/cleaner)
$ lightningcss --version
1.16.0
This is blocking using the testers.testVersion for Nixpkgs @ NixOS/nixpkgs#191453 because the version does not match the release tag version.