Hm. Good points. Nested paragraphs is possible for things like tables. E.g.:
<table><li><p><table><li><p></p></li></table></li></table>

At this point, I don't have a clear idea what the correct behaviour should be for that situation. I'm inclined to leave it as it was, pending a concrete bug of some type.

I'll revert the common code, as it is clear enough with the comments for now. Would you agree?