Modernizr adds classes to the <html> 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:
.multiplebgs div p {
/* properties for browsers that
support multiple backgrounds */
}
.no-multiplebgs div p {
/* optional fallback properties
for browsers that don't */
}
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.
Read more: Modernizr