My favorite (non-conforming, do-not-use) element is <xmp>. It switches the browser's parsing context to "generic raw text"; it emits any "child" markup as-is, without parsing it.
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).
So `<math><![CDATA[x<y]]></math>` will print x<7, but `<p><![CDATA[x<y]]></p>` will emit a `cdata-in-html-content` parse error (your compliant HTML parser is allowed to give up here if it's lazy) and is parsed as `<p><!--[CDATA[x<y]]--></p>` (your HTML parser will actually do this because giving up leads to poor user experience when no one else does)