Modernizr is a script that will detect native CSS3 and HTML5 features available in the current UA and provide an object containing all features with a true/false value, depending on whether the UA has native support for it or not.
In addition to that, Modernizr will add classes to the <html> element of the page, one for each cutting-edge feature. If the UA supports it, a class like cssgradients will be added. If not, the class name will be no-cssgradients. This allows for simple if-conditionals in CSS styling, making it easily to have fine control over the look and feel of your website.