lit.css is a minimal (less than 1kb minified) CSS framework to help you create responsive, pretty clean web app.
chota.css is a super light, pure CSS framework to help you create responsive, minimal, clean web pages and/or modern web apps.
Shards is a free, responsive UI toolkit which helps you create modern, mobile-friendly web apps using Bootstrap 4 framework.
Turret.css is a simple, fast extendable CSS library for generating responsive, accessible, mobile-friendly web applications.
IceCream is a dead simple and lightweight CSS/LESS framework for creating a responsive grid layout with ease.
Bijou is a lightweight and easy CSS framework for quickly and easily creating your own styles of grid layout, typography, tables, buttons and more.