I went looking for who else might benefit from this and found the same pattern across ecosystems: tools either re-implement the sort order and drift from it, or depend on Node.js/Prettier in ecosystems that don't otherwise use them.

A few examples:

• Elixir — tailwind_formatter is blocked on v4 support (100phlecs/tailwind_formatter#60) since resolveConfig was removed
• Ruby — Herb's Tailwind sorting rewriter uses an extracted JS package, so it can't work in tailwindcss-rails projects that have no Node runtime (marcoroth/herb#858)
• Go — templ's templ fmt doesn't sort Tailwind classes (a-h/templ#1263)
• Rust — Rustywind has an open PR trying to match the Prettier sort order; Biome has an open issue too
• Python — djLint, the main Django/Jinja formatter, has no Tailwind sorting; the Prettier plugin can't handle Jinja2
• PHP — No pure-PHP Tailwind sorting exists; Laravel Pint, PHP-CS-Fixer, and PHP_CodeSniffer have no support for it
• Editors — Outside VS Code, Tailwind class sorting either requires Node.js (tailwind-sorter.nvim) or isn't available at all

The implementation is small — a readline loop around the existing design system.

Related: in discussion #19092, @Princesseuh asked about running the CLI as a persistent process for a Rust-based static site generator, and @philipp-spiess replied:

We'd actually love to enable more use cases like this but the caveat here is that the Core API is not considered stable for now – but you'd be invited to help us shape it!

This proposal is a small step in that direction — exposing one specific design system capability (class sorting) over stdin/stdout so non-JS tools don't need to embed Node or reimplement the logic.

yo, this would be massive for non-node stacks. keeping the sort order in sync across elixir/ruby/etc manually is a nightmare. even if the core api is still shifting, a basic stdin/stdout wrapper would be a huge dx win. +1 for this! 🚀

The canonicalize subcommand and --stream support (#19796) both landed in v4.2.2. Thanks Robin for both. Streaming keeps the CLI alive as a long-running process, which is what tools that process class attributes one at a time need.

This enabled canonical_tailwind, an Elixir formatter plugin that canonicalizes Tailwind classes in HEEx templates via mix format. It delegates to canonicalize --stream under the hood.

What I was originally asking for here is addressed. Closing this out.