HTML5-Enable Every Browser: Webshims Lib
In order to HTML5-enable older browsers or to create a fallback for unsupported features, it is very common to use polyfills (like using a Flash video player if there is no HTML5 video support).
It already includes many polyfills like JW Player for audio-video fallback, ExCanvas for canvas support and more. Also, it is modular which means new pollyfills can be added with ease.
The library is very helpful to keep the page sizes small by only loading the required polyfills for incapable browsers.