Specialized

• System.css - Design system for building retro Apple interfaces.
  Repo (⭐2.6k) | #PostCSS

General Purpose

• U.S. Web Design System - Components used by U.S. federal government websites.
  Docs, Repo (⭐6.9k) | #SCSS

General Purpose

• Stacks - UI components used by Stack Overflow.
  Docs, Repo (⭐630) | #LESS

Material Design

• Beer CSS - Build material design interfaces in record time without stress for devs.
  Repo (⭐1.8k) | #CSS

Class-less

• Simple.css - Lightweight, classless CSS framework.
  Demo, Docs (⭐4.6k), Repo (⭐4.6k) | #CSS

Material Design

• Materialize - Responsive front-end library based on Google's Material Design.
  Docs, Repo (⭐949) | #SCSS

Specialized

• XP.css - CSS library for building interfaces that look like Windows XP.
  Repo (⭐2.5k) | #PostCSS

• 7.css - CSS library for building interfaces that look like Windows 7.
  Repo (⭐1.8k) | #PostCSS

Stalled Development

• CSS Remedy - Start your project with a remedy for the technical debt of CSS.
  | #CSS

Base / Reset / Normalize

• modern-normalize - Normalize browsers' default style.
  Repo (⭐6.7k) | #CSS

General Purpose

• Pico.css - Elegant styles for all native HTML elements, dark mode automatically enabled.
  Demo, Docs, Repo (⭐15k) | #SCSS

• Vanilla Framework - Simple, extensible CSS framework. Written by the Canonical Web Team.
  Demo, Docs, Repo (⭐896) | #SCSS

• PatternFly - UI framework for enterprise web applications.
  Demo, Docs, Repo (⭐737) | #SCSS #LESS

Specialized

• Tufte CSS - Style your webpage like Edward Tufte’s handouts.
  Repo (⭐6.1k) | #CSS

Stalled Development

• inuitcss - Extensible, scalable, Sass-based, OOCSS framework for large and long-lasting UI projects.
  Repo (⭐1.9k) | #SCSS

• turretcss - Responsive front-end framework for accessible and semantic websites.
  Demo, Docs, Repo (⭐800) | #CSS

Utility-based

• Open Props - CSS custom properties to help accelerate adaptive and consistent design.
  Demo, Docs, Repo (⭐5k) | #CSS #PostCSS

Stalled Development

• Propeller - Front-end responsive framework based on Google's Material Design standards & Bootstrap.
  Docs, Repo (⭐1.1k) | #SCSS

Base / Reset / Normalize

• Natural Selection - Collection of best-practice CSS selectors.
  | #CSS

Class-less

• MVP.css - Minimalist stylesheet for HTML elements.
  Docs, Repo (⭐5.1k) | #CSS

General Purpose

• Fomantic-UI - Build beautiful websites fast, with concise HTML, intuitive javascript, and simplified debugging.
  Docs, Repo (⭐3.6k) | #LESS

Specialized

• TuiCss - Library to create MS-DOS interfaces.
  Demo (⭐1.7k), Docs (⭐1.7k) | #SCSS

Stalled Development

• Semantic UI - User interface is the language of the web.
  Docs, Repo (⭐51k) | #LESS

• Tachyons - Quickly build and design new UI without writing CSS.
  Demo, Docs, Repo (⭐12k) | #CSS

Stalled Development

• modern-css-reset - Tiny little reset that you can use as the basis of your CSS projects.
  Repo (⭐3k) | #CSS

Class-less

• sakura - Minimal classless CSS framework/theme.
  Demo, Repo (⭐4.2k) | #SCSS

• Tacit - CSS framework for dummies, without classes.
  Repo (⭐1.7k) | #SCSS

Stalled Development

• Water.css - Just-add-CSS collection of styles to make simple websites just a little nicer.
  Repo (⭐8.4k) | #CSS

General Purpose

• Blaze UI - Open source modular toolkit providing great structure for building websites quickly with a scalable and maintainable foundation.
  Demo, Docs, Repo (⭐1.6k) | #SCSS

Base / Reset / Normalize

• ress - Modern CSS reset.
  Repo (⭐2k) | #CSS

General Purpose

• Cirrus - Component and utility centric SCSS framework designed for rapid prototyping.
  Demo, Docs, Repo (⭐1.4k) | #CSS

Specialized

• NES.css - NES-style CSS Framework.
  Demo, Docs (⭐21k), Repo (⭐21k) | #SCSS

Stalled Development

• Concise CSS - Give up the bloat. Stop tripping over your classes. Be concise.
  Docs, Repo (⭐940) | #SCSS

• Responsive Boilerplate - Powerful, accessible, developer friendly framework for building responsive websites.
  Docs, Repo (⭐869) | #SCSS

General Purpose

• Foundation - The most advanced responsive front-end framework in the world.
  Demo, Docs, Repo (⭐30k) | #SCSS

Stalled Development

• Bourbon - Lightweight Sass tool set.
  Docs, Repo (⭐9.1k) | #SCSS

General Purpose

• Carbon Components - Component library behind IBM's Carbon Design System.
  Docs, Repo (⭐8.3k) | #SCSS

General Purpose

• HiQ - Simple CSS foundation with responsive typography and input styling.
  Docs, Repo (⭐318) | #PostCSS

Very Lightweight

• Pure - Set of small, responsive CSS modules that you can use in every web project.
  Demo, Docs, Repo (⭐24k) | #CSS

• Picnic CSS - Lightweight and beautiful library.
  Demo, Docs, Repo (⭐3.9k) | #SCSS

• Chota - Really small CSS framework.
  Demo, Docs, Repo (⭐1.5k) | #CSS

General Purpose

• Bootstrap - The most popular HTML, CSS, and JavaScript framework for developing responsive, mobile first projects on the web.
  Demo, Docs, Repo (⭐172k) | #SCSS

• Bulma - Modern CSS framework based on Flexbox.
  Demo, Docs, Repo (⭐50k) | #Sass

• UIkit - Lightweight and modular front-end framework for developing fast and powerful web interfaces.
  Demo, Docs, Repo (⭐18k) | #LESS #SCSS

• Primer - CSS framework that powers GitHub's front-end design.
  Docs, Repo (⭐13k) | #SCSS

Material Design

• Material Components Web - Modular and customizable Material Design UI components for the web.
  Demo, Docs, Repo (⭐17k) | #SCSS

Utility-based

• Tailwind CSS - Utility-first CSS framework for rapid UI development.
  Docs, Repo (⭐87k) | #CSS

Specialized

• 98.css - CSS library for building interfaces that look like Windows 98.
  Repo (⭐10k) | #CSS

• Gutenberg - Modern framework to print the web correctly.
  Demo, Docs (⭐4.9k) | #SCSS

• Bojler - Email framework for developing responsive and lightweight email templates.
  Docs, Repo (⭐1k) | #SCSS

Stalled Development

• sanitize.css - The best-practices CSS foundation.
  Repo (⭐5.3k) | #CSS

• minireset.css - Tiny modern CSS reset.
  Repo (⭐2.8k) | #Sass

Stalled Development

• unsemantic - Fluid grid for mobile, tablet, and desktop.
  Demo, Docs, Repo (⭐1.4k)

• Centurion - Web-based framework for rapid prototyping and building larger web projects.
  Docs (⭐325), Repo (⭐325)