The idea for @tailwindcss came to @adamwathan while he was working on other SaaS ideas:

Digest - "Delicious for teams"
Kitetail - competitor to gumroad for selling digital products

He was building in public, and people kept asking him about his CSS.

https://adamwathan.me/tailwindcss-from-side-project-byproduct-to-multi-mullion-dollar-business/