You are missing the point: I know that you could add `<!DOCTYPE html>` to make that document valid and you know as well. But whoever writes the second snippet does not know because we are not there to point it out. And if you point it out they will look at you puzzled: "You are saying that it is not valid, but it renders, and in exactly the same way! Why are you making all this fuss about this "validity" thing?"
