Doing it by usergroup is problematic. It's rather awkward if someone quotes a post containing HTML, and in their post the HTML doesn't work (but in the one they quoted it did). Similarly, if someone gets promoted, what happens to their old posts? What if someone without the right to post raw HTML included, in some remote old post, some text like "<script>do_something_evil();</script>" as an example of how an attack might work, or who knows what, in the expectation that it would get escaped ― and they then got promoted to admin? What if someone makes a long post, cleverly buries some malicious HTML in it (perhaps hidden using font coloring or similar), and then gets an admin to quote it?
Having raw HTML only per forum is much, much safer. If you need more flexibility, use custom BB codes (for instance, that works great for embedding videos) and possibly extensions of various kinds