A responsive css framework can save you a lot of time for specifying rules for different screen sizes and devices. Here is a list of common css frameworks that most websites are using today.
Bootstrap is 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 modern CSS framework based on Flexbox
Semantic is a development framework that helps create beautiful, responsive layouts using human-friendly HTML.
You should use Skeleton if you're embarking on a smaller project or just don't feel like you need all the utility of larger frameworks. Skeleton only styles a handful of standard HTML elements and includes a grid, but that's often more than enough to get started.
Milligram provides a minimal setup of styles for a fast and clean starting point. Just it! Only 2kb gzipped! It's not about a UI framework. Specially designed for better performance and higher productivity with fewer properties to reset resulting in cleaner code.
All of them are open source. Don't forget to contribute if you fall in love with them.
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