Bootstrap - Simple and flexible HTML, CSS, and JS for popular UI components and interactions
Gridlex - Simple. Robust. Responsive. Flexible.
Elastic UI - React-based design library made for use with Elastic products
Semantic UI - A UI Component library implemented using a set of specifications designed around natural language
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.
Next.js - A small framework for server-rendered universal JavaScript apps