CSS Reference : Learn by example: cssreference.io is a free visual guide to CSS. It features the most popular properties, and explains them with illustrated and anima...
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 ...
CSSdoc : This is a draft to bring automatic documentation to CSS files, like its known for JavaScript and PHP files through JavaDoc/DocTags.
idiomatic-css : Principles of writing consistent, idiomatic CSS
