This addon register Modernizr into Plone's resource registry
Modernizr is a script that detects native CSS3 and HTML5 features available in the current UA and provides an object containing all features with a true/false value, depending on whether the UA has native support for it or not.
Modernizr will also add classes to the element of the page, one for each feature it detects. 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 your CSS, giving you fine control over the look & feel of your website.