> What you are looking for, I would rather call "chromeless". The browser is there, but it has no visible UI-components outside of the webpage.

Why "chromless" and not just "Full screen"? The latter is what that mode has been known by for decades? Or is it a subtle distinction that "Full screen" mode can show the location bar by moving the mouse to that edge of the screen (at least in Firefox 55), while "chromless" never lets you see the location bar?