HTML5 technologies are exciting, and can be extremely powerful, but with power, comes responsibility. HTML5 Please offers useful guidance about which parts of HTML5 are safe to use, which require use with poly-fills, and which should be avoided. VERY useful!
Really useful, up to date compatibility tables for support of HTML5, CSS3, SVG and more in desktop and mobile browsers. If you want to know if it's safe to use any of the emerging HTML5, CSS3 etc features, then a good place to start is by checking at CanIUse!