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By Dave Sussman
Web Site Performance is more complex than you think. I've been banging on about CSS for years, and how it can improve performance, but the modern world is more complex; not only do we have multiple desktop browsers, but also multiple mobile ones. The web is inherently mobile these days, so performance is more important than it was, and while you may think media queries solve your problems (I did) you'd be wrong; CSS can still cause performance issues, simply because of the loading and parsing of the file. Here's a good little article that explains a few issues you might not have thought of; I hadn't.
http://www.igvita.com/2012/06/14/debunking-responsive-css-performance-myths/