Setup: what the template generates

npm create shadow-app@latest my-app -- --tailwind adds Tailwind support and writes the configuration shown below.

To add Tailwind to an existing ShadowJS app, install tailwindcss and @tailwindcss/vite, add tailwindcss() after shadow() in plugins, create the CSS file with the @import "tailwindcss"; line above and import it once from main.tsx. The shadow() plugin only compiles .jsx and .tsx files, so CSS goes straight to the Tailwind plugin.