Kube is a lightweight CSS framework including many useful components – from grids to modals – and focusing on vertical rhythm and typography.
Simple Grid is another mini CSS framework providing a responsive 12-column grid system and some extra classes for getting started with typography.
Simple Grid: Lightweight and responsive CSS grid
Spectre.css is a lightweight CSS framework for building responsive and websites. It is based on flexbox and provides many useful components.
Spectre.css: A lightweight CSS framework based on flexbox
Milligram is a minimal and lightweight CSS framework to build websites. It is ideal for front end developers that need a design agnostic framework.
Milligram: Minimal CSS framework
Bulma is a modern, lightweight and design agnostic CSS framework based on Flexbox. Released by jgthms.
Bulma: CSS framework based on Flexbox
Photon is an UI toolkit for building full featured desktop apps with Electron using simple HTML and CSS.
Photon – Desktop app framework
Material Design Lite is a lightweight framework based on Google Material Design. It doesn’t rely on any JavaScript frameworks.
Material Design Lite framework
MUI is a lightweight HTML/CSS/JS framework that follows Google’s Material Design guidelines.
MUI – Material Design framework
Semantic UI is a development framework based on user interface language you can use to build websites. It’s a great alternative to Bootstrap.
Semantic UI framework
Materialize is a modern responsive front-end framework based on Material Design by Google.
Materialize – Material Design framework
Webplate is a new front-end framework bringing the best of the web together into one great package. It includes some free plugins.