CSS reference into HTML doesn't reflect

I have CSS Web resource with below configuration.

Name:-  Prefix_Name.css
DisplayName:- Name.css
URL:-  http://url/Prefix_Name.css

when I use below line in html web resource, it doesn't relect the css font/size/other gui??

<link href="css/Prefix_Name.css" rel="stylesheet" type="text/css" />

 So some how, HTML doesn't able to refer css at all?? I tried few VS 2012 package approach and also tried few path,. but doesn't work.  Any idea, how to refer the css correctly here??

Thanks,

Nick