HTML5Rocks: HTML5-CSS3 Resource By Google
HTML5Rocks is an online resource by Google that regularly shares various resources on HTML5 and CSS3.
The site has very detailed tutorials that digg stuff like HTML5 video or @web-fonts which are good starting points on the subjects.
An online playground with shortcuts to JS APIs + HTML-CSS properties helps trying new codes and seeing the outputs instantly.
And there is HTML5 Studio, which is a showcase of HTML5 codes like page-flip, video player, carousel, geolocation and more that are open source. It is possible to get inspired from them or re-use them in our projects.