The nifty thing about this Tailwind "JIT" starter for Eleventy is that you can see your changes take effect while you type into your templates, making it much easier to ask yourself, "How would it look if I changed mx-2 to mx-4?" when you're totally new to Tailwind and can't imagine it all from memory.
(You might have to play with /src/tailwind/tailwind.config.js if you're using, say, .11ty.js templates, so as to add them to the "watched" files.)
Many thanks to Darrik Moberg for the code on which this is modeled, and for all the debugging help adapting it to Tailwind JIT. I can't really take credit for this code -- just for stripping it down to a starter.
Thanks also to Mike Allanson for additional troubleshooting.
Known issue: every once in a while, particularly as you start overwhelming Tailwind JIT with things like plugins and a larger template codebase, your changes might stop taking effect on the screen served by your running development server (you can see that things stop getting freshly written out to /dist/tailwindoutlive.css).
Solution: restart your server once or twice. Now that Tailwind JIT builds quickly, rather than taking up to a minute like legacy Tailwind, you should be fine, other than it being a bit of a nuisance.
Live example at https://11tytailwindjit.netlify.app/