I don't think linkchecker actually cares about the document type, so I'm not sure why we're even testing this...
Shown every time linkchecker is run: /usr/lib/python3.7/site-packages/bs4/element.py:16: UserWarning: The soupsieve package is not installed. CSS selectors cannot be used. 'The soupsieve package is not installed. CSS selectors cannot be used.'