<%@ Page Language="C#" 
         AutoEventWireup="true" 
         CodeFile="SomeXAML.aspx.cs" 
         Inherits="SomeXAML"
         ContentType="text/xaml" %>
<%@ Register Src="~/XamlButton.ascx" TagPrefix="xaml" TagName="Rectangle" %>
<Canvas xmlns="http://schemas.microsoft.com/client/2007"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Width="400" Height="400">
  <xaml:Rectangle runat="server" />
  <xaml:Rectangle Top="50" Left="50" 
                  Width="75" Height="75"
                  FillColor="Red" 
                  runat="server" />
</Canvas>          
