Currently our Sanitizer is only capable of sanitizing attributes. We cannot sanitize whole tags (besides just making them not work) so we can't clean up invalid elements like <center>, <font>, and <strike>.
We need to update our Sanitizer code so we can also substitute whole tags. And we'll need to make sure we also replace the closing tag.
This will probably mean creating a new set of functions to call, deprecating the old ones, and then fixing the parser to use the new functions. This involves whole element replacement so it might not be as trivial to do in the parser as attributes were.