Open source. Open possibilities.

Oat is an ultra-lightweight, zero-dependency semantic HTML/CSS/JS UI library (~10KB min+gz). It needs no framework or build step, styles semantic tags contextually without classes, and uses minimal JS WebComponents for dynamic parts.

Bulma is a modern, Flexbox-based CSS framework distributed as a single CSS file with no JavaScript. It offers Sass sources for variable customization and works with any frontend stack.

Numl is a deprecated UI design language, web-component library and runtime CSS framework for quickly building interfaces that follow a design system, loadable with a single JavaScript module and no bundler or config.

Spruce CSS is an open-source, lightweight Sass-based CSS design system and framework offering variables, mixins, functions, few utility classes and CSS custom property theming with dark mode.

A lightweight, modular and customizable design system built with vanilla CSS, using CSS variables, mobile-first responsive principles and accessibility practices. It includes examples, design tokens, build and lint scripts, format conversion, and a CDN-ready combined CSS file.

Bootstrap is a widely used open-source front-end framework for building responsive, mobile-first websites. It provides compiled and source CSS, JavaScript components, utility classes, and theming tools, with options for install via npm, Composer, NuGet and more.

Tailwind CSS is an open-source utility-first CSS framework for rapidly building custom user interfaces. The repository points to tailwindcss.com for full documentation, GitHub Discussions for community help and ideas, and contributing docs for pull requests.

A curated directory of CSS frameworks organized by category, including base resets, classless frameworks, lightweight options, general-purpose frameworks, Material Design, utility-based, specialized, and stalled projects.

Oruga is a lightweight Vue.js 3 UI component library with no CSS framework dependency. It provides customizable components, tree-shaking, themes, and class-mapping configuration for integrating Tailwind, Bulma, Bootstrap, or custom styles.

daisyUI is a free and open-source component library for Tailwind CSS. It offers pre-designed, customizable UI components that can be used directly in HTML. The project is popular and includes extensive documentation, a component gallery, and community support.

Materialize is a modern CSS framework based on Material Design principles, providing a set of tools to create responsive and visually consistent user interfaces.