HTML5 Please is a helpful list of HTML5 and CSS3 features, and includes information about a) how well each feature is supported by modern browsers and mobile devices; b) whether the feature is largely considered ‘ready to use’; and c) whether the feature required fallbacks or polyfills, and even has links to what are considered the best fallbacks currently available.
You can browse the full list, or sort by several options, including browser or mobile support, and by the code you can use as is, or the code you should — for the time being — avoid.
So far I’ve been sticking to the HTML5 and CSS3 tricks I feel require no fallback (hello, border-radius!). With a solid resource like HTML5 Please, I’m running out of reasons not to extend my front-end development skills on future projects.
HTML5 Please is a community project; if you are already well into the world of websockets and
<canvas>, you can contribute on GitHub.