<plaintext> is also fun. Unlike <xmp> you can't get out of it with a closing tag.

Aside: plaintext is one reason browser DOM is a superset of HTML. By which I mean you can add <plaintext> to the middle of your DOM through JavaScript and everything is hunky-dory, but it's impossible to serialize the DOM to HTML so it'll HTML-parse back to the same DOM (at least if the serializer doesn't produce any JavaScript).