Hacker News Reader

Well, sure, just like I can create my invented domain name like hacker.news and use my own IP addresse 1.2.3.4 without being allocated them. The Internet interoperates by everyone agreeing to follow the same agreed conventions, but there is no rule that says you have to. The registry for HTTP status codes has no 420 code and could be assigned for a different purpose in the future: http://www.iana.org/assignments/http-status-codes

Yes, they can respond with 420. However, my browser does not have a predetermined response for that. Whereas if they respond with 200 or 302 or 404, all browsers know what to do; that's what it means to be a "standard".

> HTML has a DTD

Tangentially to your point, that's only true of the 4.x and earlier versions of HTML which are SGML applications, WHATWG HTML / W3C HTML5 is not SGML-based and does not have a DTD.