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.
Svelte - Cybernetically enhanced web apps
Bootstrap - Simple and flexible HTML, CSS, and JS for popular UI components and interactions
Glimmer - Fast and light-weight UI components for the web
Materialize CSS - A modern responsive front-end framework based on Material Design
Preact.js - Preact is a fast 3kB alternative to React with the same modern API. Components & Virtual DOM.