In a previous article, I described what could cause this error. While helping a customer the other day, we discovered another culprit, password protecting a directory. The solution is to put the resources (i.e. .css, .js, image files, etc.) in a location which is publicly accessible (i.e. not under directory tree which has been password protected). I also think this error may be particular to Chrome’s Developer Tools. The customer was using Internet Explorer, and this was the error that was thrown instead:
SEC7113: CSS was ignored due to mime type mismatch.