Using TailWind is a bit tricky, there are several blog/forum posts about it, because Frontity uses Styled Components. I tried some of the methods such as using the twin.macro library however this requires wrapping your CSS class strings in the tw` ` string processor - I don't particularly like styled components and this seemed pretty inconvenient.
My solution: to use tailwind by importing a .css file as a Frontity <Global> style, with a separate npm script to monitor for file changes and run the tailwind build accordingly, with the Frontity dev script also watching for changes and rebuilding again whenever the constructed .css file changes.
It's a bit janky because you get 3 builds whenever you make a style change: a Frontity build change because a .js file changed, a tailwind build because you changed a .js file (which the tailwind build script is watching for), and then another Frontity build because the tailwind-generated .css file in the src directory has changed. I like this method though because I only need the Tailwind library and nodemon to get this to work.