Ah. Minidom does include it, but browsers don't (Chrome and FF). I did this because there's no DOM way to stringify the `DocumentType` node.

Options:
1. Remove it from minidom and add a utility to stringify it.
2. Go with it as-is for the moment and update when I do 1.
3. Remove the minidom changes from this PR. They are not essential (but it does make it *a lot* easier to install).