Even though I use Tailwind CSS for larger projects, there are smaller, self-contained cases where Pico CSS is a perfect fit.
Their "Usage scenarios" page describes it spot on:
I discovered Pico CSS just last week, and it turned out to be exactly what I needed for a small Hugo theme (govanity, vanity URLs for Go modules/packages with Hugo: https://github.com/foundata/hugo-theme-govanity). From discovering Pico, reading the docs, and integrating it, I was done in about two hours.