If you’re designing an XML-based data format, then I beg you, please read the few following rules and obey them. XML may look easy, and even is easy but that doesn’t mean that writing a good one is. And if you’re going to invent second HTML, then please, just use JSON or any other random container. That will be easier for you, and easier for us.
Optional runtime dependencies (or «suggested dependencies») are one of the late problems we’re facing in Gentoo. There’s definitely a need for some standard solution, and it’d be great to put it in the next EAPI. Sadly, there’s no consensus how to solve it.