Animate.css — Animate.css is a bunch of cool, fun, and cross-browser animations for you to use in your projects. Great for emphasis, home pages, sliders, and general just-add-water-awesomeness.
Bootstrap - The most popular HTML, CSS, and JavaScript framework for developing responsive, mobile first projects on the web.
Bulma — Bulma is a modern CSS framework based on Flexbox.
Codrop — An extensive CSS reference made by codrops. It contain with all the important properties and info to learn CSS from the basics
Foundation — The most advanced responsive front-end framework in the world. Quickly create prototypes and production code for sites that work on any kind of device.
Materialize — A modern responsive front-end framework based on Material Design
PostCSS — Transforming styles with JS plugins. These plugins can lint your CSS, support variables and mixins, transpile future CSS syntax, inline images, and more.
Pure — A set of small, responsive CSS modules that you can use in every web project.
Semantic UI — Semantic is a development framework that helps create beautiful, responsive layouts using human-friendly HTML.
Tachyons — Functional css for humans. Quickly build and design new UI without writing css.
Tailwind CSS — A utility-first CSS framework for rapid UI development.
UIkit — A lightweight and modular front-end framework for developing fast and powerful web interfaces.