What is Modernizr?
It’s a collection of superfast tests which run as your web page loads, then you can use the results to tailor the experience to the user.
Modernizr makes it easy to deliver tiered experiences: make use of the latest and greatest features in browsers which support them, without leaving less fortunate users high and dry.
It used by many popular websites including Twitter, USMagazine, Good.is, About.com, etc.
Video for How to use Modernizr