Taking advantage of the new capabilities of HTML5 and CSS3 can mean
sacrificing control over the experience in older browsers. Modernizr 2
is your starting point for making the best websites and applications
that work exactly right no matter what browser or device your visitors
use.

Thanks to the new Media Query tests and built-in YepNope.js
micro-library as Modernizr.load(),you can now combine feature
detection with media queries and conditional resource loading. That
gives you the power and flexibility to optimize for every
circumstance.

它有很多内置的方法来测试浏览器功能，并提供了一种有用的方式来提供回退代码，以便在不支持要使用的功能时。

更多信息：http://www.modernizr.com/