Bootstrap - Simple and flexible HTML, CSS, and JS for popular UI components and interactions
w2ui - W2UI is a small JavaScript UI library with a complete set of widgets: layout, grid, sidebar, toolbar, tabs, fields, popup, utilities.
Materialize CSS - A modern responsive front-end framework based on Material Design
Svelte - Cybernetically enhanced web apps
Tailwind CSS - A utility-first CSS framework for rapidly building custom user interfaces.
Preact.js - Preact is a fast 3kB alternative to React with the same modern API. Components & Virtual DOM.