The first thing you need to do is read the HTTP spec which will explain what you can expect to receive over the wire. The data returned inside the content will be the "rendered" web page, not the source. The source could be a JSP, a servlet, a CGI script, in short, just about anything, and you have no access to that. You only get the HTML that the server sent you. In the case of a static HTML page, then yes, you will be seeing the "source". But for anything else you see the generated HTML, not the source.
When you say modify the page and return the modified page
what do you mean?