Guide to CSS support in email clients : Designing an HTML email that renders consistently across the major email clients can be very time consuming. We've put together this guide to save you...
idiomatic-css : Principles of writing consistent, idiomatic CSS
Email Standards Project : The Email Standards Project is about working with email client developers and the design community to improve web standards support and accessibility ...
Write CSS without worrying that overzealous, pre-existing styles will cause problems. MaintainableCSS is an approach to writing modular, scalable and of course, maintainable CSS.