Our stylesheet starts like this:

@import "tailwindcss" ; @config "../tailwind.config.ts" ;

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Line one is Tailwind v4. Line two reaches back and loads an 80-line v3-shaped JavaScript config: content globs, a prefix , darkMode , a theme.extend , two keyframes and a require() d plugin. It works, because @config exists precisely so that a project can move to v4 without rewriting its theme the same afternoon. The question I had not asked is which parts of that file still decide anything.

The useful thing about this question is that it is answerable from outside.…