If you’ve been looking to implement modern browser features right now then the Modernizr javascript framework is indispensable:
Modernizr is a small and simple JavaScript library that helps you take advantage of emerging web technologies (CSS3, HTML 5) while still maintaining a fine level of control over older browsers that may not yet support these new technologies.
The “What Modernizr Doesn’t Do” section might make its purpose more clear:
Modernizr does not add missing functionality to browsers; instead, it detects native availability of features and offers you a way to maintain a fine level of control over your site regardless of a browser’s capabilities.
Here’s an example taken from Mark Pilgram’s Dive Into HTML5 book:
if (!Modernizr.inputtypes.number) { // no native support for type=number fields // maybe try Dojo or some other JavaScript framework }