Responsive Email Framework
"MJML is a markup language designed to reduce the pain of coding a responsive email. Its semantic syntax makes the language easy and straightforward while its rich standard components library shortens your development time and lightens your email codebase.
MJML’s open-source engine takes care of translating the MJML you wrote into responsive HTML. It's responsive by design on most-popular email clients, even Outlook.
MJML is based on React."