Bulma - Bulma is an open source CSS framework based on Flexbox and built with Sass. It's 100% responsive, fully modular, and available for free.
Foundation - The most advanced responsive front-end framework in the world
Bootstrap - Simple and flexible HTML, CSS, and JS for popular UI components and interactions
Geb - Browser Testing, Testing, API, Web Development, Software Testing Tool, and Rest
Materialize CSS - A modern responsive front-end framework based on Material Design
Skeleton CSS - A Beautiful Boilerplate for Responsive, Mobile-Friendly Development