Alas, your six bytes would drop older browsers into quirks mode, which makes is not nice. The only reason HTML5 has doctype at all is because it forces browsers

Alas, your six bytes would drop older browsers into quirks mode, which makes is not nice. The only reason HTML5 has doctype at all is because it forces browsers into standards mode. What you propose sounds a bit like the way XHTML2 was intended to go, we know how that ended. There is and will be tons of invalid documents on the web—the wast majority of it, so browser rejecting them has no future, the blame will be on the browser, not the authors. Having at least some consistency in dealing with invalid documents is A Good Thing™.