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 <body>
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.
http://www.modernizr.com