The whole point of vanilla HTML is that it has few presentation details. It has some headings, bold, italic, and such. If the page does not specify margins or font size, the browser absolutely should set these so it is most readable on the device. If I write plain HTML today I am not optimizing for some VR headset that will be used twenty years hence. The headset should render the plain HTML in a manner faithful to the semantic markup, not so it looks the same way it looked on Netscape with a VGA screen.
