how to write aspx page html from aspx.cs page on page load

i want to write aspx page html from aspx.cs page on page load..
i hav already used  div.innerHtml...
i want to write below code in aspx page from aspx.cs page
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled Document</title>
<link href="css.css" rel="stylesheet" />
</head>
 
<body>
        <div>
 
        </div>
</body>
</html>
...