An interesting framework that I saw here on HN a couple of days ago is Tailwind UI (https://tailwindui.com/). It's not out just yet but looks very promising.

Other than that I would recommend Bulma (https://bulma.io/). It's like bootstrap but with a different philosophy on how CSS should be implemented. Also, it doesn't include any javascript. I think that was a good decision on their part as you won't have to fight the default behaviors on their components if you want to do something slightly different.