During the course of a project we often end up adding elements to style sheets which do not get used. This causes css files to bloat which impacts performance. The best thing to do is to keep track of css elements which are not being used and remove them. This can become a strenuous task especially if you have a large website. But there is a tool which can be used to find unused css. The tool is called Dust-Me Selectors and it is an addon for Firefox.  

Running the tool on One .Net Way gives me this result. There are 131 selectors which are not being used at present. This should however be taken as a starting point for clean up. You must always manually check if css is being used in client side scripts or server side code.

Download link for Dust-Me Selectors.