Twitter Bootstrap – Web Application Framework
Bootstrap is the most popular HTML, CSS, and JS framework for developing responsive, mobile first projects on the web. Bootstrap is open source. It’s hosted, developed, and maintained on GitHub.
Bootstrap (currently v5.1.3) has a few easy ways to quickly get started, each one appealing to a different skill level and use case. Read the getting started guide to see what suits your particular needs.
Foundation – Responsive Framework
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.
Foundation doesn’t require any particular back end. In fact, Foundation has been used with dozens of back-end and CMS technologies, and Foundation is used by hundreds of thousands of sites every day to deliver rock-solid responsive sites.
YAML 4 – Modular CSS Framework
YAML was released in October 2005 as a base for developing flexible layouts, with a strong emphasis on meeting the challenges that result from working with flexible (elastic, fluid) containers and varying units. YAML has quickly matured into a stable and versatile layout framework, and is regularly updated. Third party developers have created integration tutorials and templates for a long list of Content Management Systems.
YAML provides a complete set of matched building blocks to create complex websites. Grids, navigation, forms, typography module and all provided add-ons work seamlessly together.
(For the free use of the YAML framework, a backlink to the YAML homepage (https://www.yaml.de) in a suitable place (e.g.: footer of the website or in the imprint) is required. If you are a commercial software developer and you want to release your software under a different license than CC-BY 2.0, you may purchase commercial licenses.)
Semantic UI
Semantic is a development framework that helps create beautiful, responsive layouts using human-friendly HTML. Semantic UI treats words and classes as exchangeable concepts. Semantic comes equipped with an intuitive inheritance system and high level theming variables that let you have complete design freedom.
Semantic is designed completely with em making responsive sizing a breeze. Design variations built into elements allow you to make the choice how content adjusts for tablet and mobile.
Semantic UI is a free open source project (MIT License).
Pure by Yahoo!
Pure is a set of small, responsive CSS modules that you can use in every web project. Pure is ridiculously tiny. The entire set of modules clocks in at 4.0KB minified and gzipped. Crafted with mobile devices in mind, file sizes are kept small and every line of CSS was carefully considered. If you decide to only use a subset of these modules, you’ll save even more bytes.
Pure is responsive out of the box, so elements look great on all screen sizes. By using Grids, Menus, and more, it’s easy to create beautiful responsive layouts for all screen sizes. Unlike other frameworks, Pure’s design is unopinionated, minimal and flat. By adding a few lines of CSS, you can customize Pure’s appearance to work with your web project.
UIkit by YOOtheme
UIkit is a lightweight and modular front-end framework for developing fast and powerful web interfaces. It gives you a comprehensive collection of HTML, CSS, and JS components which is simple to use, easy to customize and extendable.
UIkit is developed in LESS & SASS to write well-structured, extendable code which is easy to maintain. UIkit is absolutely free of charge and you can use, copy, merge, publish and distribute the framework without any limitations. (Licensed under MIT license.)