Bootstrap 5 & Material Design UI KIT

Primer is GitHub's design system. This is the CSS implementation

Official Sass port of Bootstrap 2 and 3.

CSS3 Animations with special effects

Standards for developing consistent, flexible, and sustainable HTML and CSS.

The Less Formal CSS Framework

Pre-grid responsive layout toolkit for Sass, now deprecated

The front-end CSS framework for building experiences for Office and Microsoft 365.

The GTK Stylesheet for elementary OS

A lightweight responsive Sass/CSS framework based on flexible box.

💄 CSS UI kit with BEM convention and SASS