I assume, thats you master page markup, so try it like this:
<head runat="server">
<title></title>
<link href="/assets/css/layout.css" rel="stylesheet" runat="server" />
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.8.2/jquery.min.js"></script>
<script src="//use.edgefonts.net/source-code-pro.js"></script>
<script src="/assets/scripts/modernizr.js"></script>
<asp:ContentPlaceHolder ID="cph_head" runat="server">
</asp:ContentPlaceHolder>
</head>
The asp:ContentPlaceHolder
should stay empty at your master page. it is overridden by what is on you child pages.