This online guide is a great reference point to exploring which features to use in HTML5; which browsers support which tags, which you should avoid for now, which need fallbacks, etc.
From the Site:
- supported by IE10+ IE9+ IE8+ IE7+ no IE
- not supported by mobile devices older mobile devices
- requiring prefixespolyfillfallback
- that you should use use with caution avoid
- that are css htmlapijs
- all features