Or perhaps I'm misunderstanding, since you mention the "fragment" method. But if you're going to provide a method to return a fragment, the limitations of whatever module you get to do the heavy lifting aren't relevant, surely.
Well, it could be, if it can't return a fragment, the behaviour has to be defined...
I would imagine it can't be too difficult to have the formatter turn an HTML document into a fragment? (Am I missing something?)
Justin