So here we're collecting all the shims, fallbacks, and polyfills in order to implant html5 functionality in browsers that don't natively support them.
The general idea is that: we, as developers, should be able to develop with the HTML5 apis, and scripts can create the methods and objects that should exist. Developing in this future-proof way means as users upgrade, your code doesn't have to change but users will move to the better, native experience cleanly.
Looking for a way to conditionally load these scripts client-side based on feature detects? See Modernizr. Looking for a guide to writing your own polyfills? See Writing Cross-Browser JavaScript Polyfills.