Actually, I'd rather leave it to the author of the document, because it seems a tough problem to solve.

At http://markdress.org/goo.gl/zTG1q, for example, the image is linked to an absolute URL: /graphics/markdown/mt_textformat_menu.png. It's going to be a bit cumbersome to traverse through goo.gl and detect that this is actually at http://daringfireball.net/. Might be easier if authors either stuck to pure relative URLs, or full URLs with domains.

But that's just me passing the buck...

A quick workaround would be to add the `<base>` tag: https://developer.mozilla.org/en/HTML/Element/base to your generated HTML and have it point it to the target URL.