Bootstrap - Simple and flexible HTML, CSS, and JS for popular UI components and interactions
Metro UI - Metro UI CSS a set of styles to create a site with an interface similar to Windows 8.
Tailwind CSS - A utility-first CSS framework for rapidly building custom user interfaces.
Materialize CSS - A modern responsive front-end framework based on Material Design
UIKit - A lightweight and modular front-end framework for developing fast and powerful web interfaces
Material Design Lite - Material Design Lite Components in HTML/CSS/JS