Whether or not overdue_notices.pl creates html output is governed by the --html option on the command line. This gives the users the impression that they can get html formatted output on a given notice by checking the 'HTML notice' check-box; they are often confused when they don't.
Sorry, that wasn't as clear as it should have been... The fact that the *only* control over html is via the command line switch causes confusion, because each notice has an html check-box, but that doesn't actually control whether or not that notice is rendered as HTML.