An open source toolkit for developing with HTML, CSS, and JS. Quickly prototype your ideas or build your entire app with our Sass variables and mixins, responsive grid system, extensive prebuilt components, and powerful plugins built on jQuery.
A Framework for any device, medium, and accessibility. Foundation is a family of responsive front-end frameworks that make it easy to design beautiful responsive websites, apps and emails that look amazing on any device. Foundation is semantic, readable, flexible, and completely customizable.
A highly customizable, low-level CSS framework that gives you all of the building blocks you need to build bespoke designs without any annoying opinionated styles you have to fight to override.
A free, open source CSS framework based on Flexbox and used by more than 200,000 developers.
Semantic UI is a modern front-end development framework, powered by LESS and jQuery. It has a sleek, subtle, and flat design look that provides a lightweight user experience.
A lightweight and modular front-end framework for developing fast and powerful web interfaces.
In the hands of the expert developer, Susy is like a flamethrower that blows everything else away. Lesser mortals, of course, will manage to burn their hands.
See here for a bigger list of CSS frameworks.