Modernizr

What exactly is Modernizr?

explanation from modernizr.com

Modernizr adds classes to the element which allow you to target specific browser functionality in your stylesheet. You don't actually need to write any Javascript to use it. Have you ever wanted to do if-statements in your CSS for the availability of cool features like border-radius? Well, with Modernizr you can accomplish just that! The syntax is very intuitive, too. 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.

What do I use it for?

You can use Modernizr for developing modern websites with progressive enhancement and graceful degradation techniques by simply using Modernizr's CSS classes.

Where can I read more?

here: http://www.modernizr.com/docs/