In order to add source code to your page, you'll have to have some code to work with. Whether it's an application that links to your newsletter or some other feature, track down the HTML, CSS or ...

Parts of Twitter’s source code, the underlying computer code on which the social network runs, were leaked online, according to a legal filing, a rare and major exposure of intellectual property ...

The source code for my "private" game page, as accessed before Monday, with the relevant game names highlighted. The potential for privacy breaches continue from there.