MailJet Markup Language (MJML) is a responsive email framework created by the team at MailJet. It’s built as a Node package which lets you code your emails in MJML. Then it will convert them into responsive HTML for your email campaigns. They’ve also built plugins for Atom and Sublime.
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. MJML’s open-source engine generates high quality responsive HTML compliant with best practices.