getting error while applying css to the master file and aspx

hello sir,   
i am trying from last week, i am not possible to apply the css to my any of the aspx files
getting problem "element html cannot be nested within the div" this is while i am trying with the put the ContentPlaceHolder within the head tag and also trying to put  the link tag
within the header so can u people please help me on this regards
thanking u in advance :)