css is a lightweight, responsive and modern CSS framework for faster and extensible development.

A lightweight and modular front-end framework for developing fast and powerful web interfaces. Created by @yootheme

Bulma is a free, open source CSS framework based on Flexbox and built with Sass. It's 100% responsive, fully modular, and available for free.

MJML is a markup language designed to reduce the pain of coding a responsive email. Its semantic syntax makes it easy and straightforward and its rich standard components library speeds up your development time and lightens your email codebase.