If for some reason you need to capture page output either to log it, analyze it or do a global string replace, then you will need to intercept the rendered Html before it makes it to the wire on the way to the client. Here is a code snippet that will do just that.
protected override void Render(HtmlTextWriter writer)
StringBuilder sb = new StringBuilder();
HtmlTextWriter htw = new HtmlTextWriter(new StringWriter(sb));
The StringBuilder will get the content, and you can do with it what you please. Then just make sure to call writer.Write( ) to render the final content to the Response stream.