Improving Responsive Web Design With RESS

RESS never became a widely-used technique because feature detection is difficult — especially on the server. Your detection code must be verified, updated and maintained every time a new browser or feature is released. Fortunately, there are third-party services such as Netbiscuits which do the hard work for you and are constantly updated with the latest device information.

http://www.sitepoint.com/improving-responsive-web-design-ress/