Bulma - Bulma is an open source CSS framework based on Flexbox and built with Sass. It's 100% responsive, fully modular, and available for free.
CSS Peeper - Smart CSS viewer tailored for Designers.
Material UI - A CSS Framework and a Set of React Components that Implement Google's Material Design
Purgecss - Easily remove unused CSS
Milligram - A minimalist CSS framework
Tailwind CSS - A utility-first CSS framework for rapidly building custom user interfaces.