Modernizr is a small, simple yet useful JavaScript library allowing you to use CSS3 & HTML5 features while maintaining control over unsupported browsers.

Modernizr uses feature detection to test the current browser against upcoming features like rgba(), localStorage, applicationCache, CSS Transitions, border-radius and many more.

Some of the features that Modernizr 1.1 detects are:

• Geolocation API
• sessionStorage
• Web Workers
• CSS Gradients
• Multiple backgrounds

With Modernizr, you can use CSS3 & HTML5 features with an easy way to control the fall-backs for non-supported browsers.

Website: http://www.modernizr.com/