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
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
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
Webplate is a new front-end framework bringing the best of the web together into one great package. It includes some free plugins.
Webplate – Free front-end framework
Bootflat is an open source flat UI kit based on Twitter’s Bootstrap 3.1 CSS framework. It provides a faster way to create elegant web apps.
Bootflat – Flat UI kit for Bootstrap
Framework7 is a powerful and full featured HTML framework for building iOS7 apps.
Framework7 – HTML framework for iOS7 apps
Ionic is a beautiful front-end framework for developing hybrid mobile apps in HTML5. Available on Github.