We just released Tailwind CSS v4.1.6 and made some improvements to the upgrade tool.
You should be able to run the upgrade tool on existing Tailwind CSS v4 projects, and keep them modern and up-to-date.
`npx @tailwindcss/upgrade@latest`
But what can it do?
If you used to have arbitrary properties like `[key:value]` just because Tailwind CSS didn't have a specific feature yet. Once we release the feature, we can upgrade these instances.