Not sure if it matches your criteria of a mature component framework, but there's Foundation and UIkit. I've used both but all of my new projects use Tailwind.

Not sure if it matches your criteria of a mature component framework, but there's Foundation and UIkit. I've used both but all of my new projects use Tailwind. It matches my needs better because I use minimal JavaScript. It also doesn't get in my way and the documentation is refreshingly clear. https://get.foundation/ https://get.foundation/ https://getuikit.com/ https://getuikit.com/ > And why aren't there more? It seems to me these tightly coupled CSS + JS component frameworks are in an awkward position these days. React/Vue allows you more flexibility. If a project is really simple you can use vanilla JavaScript or something like Alpine.js. Perhaps the demand isn't there anymore.