php-mjml is open source: our MJML renderer for PHP
MJML turns a simple markup language into responsive HTML email, usually with a Node toolchain doing the work behind it. We ported it to PHP end to end: php-mjml/php-mjml needs no Node.js, no second runtime in the deployment. It has rendered this website's emails since February; as of this week it's open under the MIT licence for anyone to use.