>In theory, HTML is device independent. In practice, we can't even get web apps to work on desktop and mobile, to the point where even sites like Wikipedia, the simplest and most obvious place where HTML should Just Work across devices, has a whole fricking separate subdomain for mobile.

Does he actually know why the cases are separate? Asserting this without any historical analysis is meaningless. For all we know, Wikipedia split desktop and mobile domains before responsive tooling in the browser got good and they just never changed it cause, like most businesses, they have better things to work on as the current solution still works.