When I first heard Google launched "Google Doctype" I thought "oh no, Google are creating their own web page DOCTYPEs?" Fortuntely the answer is no.
If you have a Google account you can also hit the Edit link, wiki style :)
I'll have to try out this reference with my next web project. I looks like it could save my time trawling through the verbose w3c specs or the poorly designed (but still useful) w3schools site.