Which subset of html does WordPress support?

WordPress supports all html imaginable. Actually, WP can produce html that is invalid or totally non-existent. Even better, you can have WP produce total gibberish if you want to, or a nasty piece of malware, or code that would only run on a 70s Unix machine. And yes, all of this can be typed in the editor.

Because WP essentially glues strings of characters from the database, templates and plugins together and sends it to the browser, no matter what these strings contain.