open that aspx page with your notepad
you might be getting some code
suppose you are getting
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="index.aspx.cs" Inherits="user_index" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title>Untitled Page</title>
</head>
<body>
<form id="form1" runat="server">
<div>
Hello World
</div>
</form>
</body>
</html>
Remove the tag
<![CDATA[<%@ Page Language="C#" AutoEventWireup="true" CodeFile="index.aspx.cs" Inherits="user_index" %>]]>
then save txt file with .html extension.
In this you will be able to see the design , but you can't see the server controls of asp.net.