I'm getting mad. Site.master page contains the following line within head tag. When the project starts I can see in firebug Net panel that this line is ignored. Even I don't get 404 not Found error;
<link href='<%=Page.ResolveUrl("~/NewStyles/ui-lightness/jquery-ui-1.8.20.custom.css")%>' type="text/css" />
What's wrong with it?