link tag require head tag.So
place a holder inside head tag in master page.
<head runat="server">
<asp:ContentPlaceHolder ID="ContentPlaceHolder2" runat="server">
</asp:ContentPlaceHolder>
Now your child page have two place holders
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder2" Runat="Server">
--your styles and scripts
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
----your contents
</pre>