Tested with firefox 3.0.3, i don't know about previous versions. For instance, try to rename style.css to style.txt and include it into html header. Firefox show no style applied, while Internet Explorer 7 had no problems including .txt styles. I don't know why is that so, is this a bug or intended behaviour. I think that styles should be included regardless file extension, otherwise header link tag type=text/css have no meaning.