There is an annoying CSS limit in IE versions 9 and below, which leads to all CSS selectors beyond the 4095th to be ignored. Due to Sass (–> more selectors) and the asset pipeline (–> bigger stylesheets) this is especially a problem in the Rails world. This blogpost gives a little bit of background on the issue and introduces the CssSplitter gem, which helps Rails developers to split up their overweight stylesheets for IE compatibility. http://railslove.com/blog/2013/03/08/overcoming-ies-4096-selector-limit-using-the-css-splitter-gem