Google Doctype
Google recently released Google Doctype.
Google Doctype is an open encyclopedia and reference library. Written by web developers, for web developers. It includes articles on web security, JavaScript DOM manipulation, CSS tips and tricks, and more. The reference section includes a growing library of test cases for checking cross-browser and cross-platform compatibility.
Looks to be an excellent resource for Web Developers. John Resig breaks it down better than I could do and tells us about two hidden additions.