MODERNIZR.JS
small JavaScript library that detects the availability of native implementations for next-generation web technologies
Unlike with the traditional—but highly unreliable—method of doing “UA sniffing,” which is detecting a browser by its (user-configurable) navigator.userAgent property, Modernizr does actual feature detection to reliably discern what the various browsers can and cannot do. After all, the same rendering engine may not necessarily support the same things, and some users change their userAgent string to get around poorly developed websites that don’t let them through otherwise.