A Webbrowser these days is simply much, much more than a static document viewer, despite this might be, what you want it to be.
And those who really want only simple HTMl rendering, I believe there are lightweight alternatives (?).
And if not, well then maybe there is simply not enough demand, because most people apparently want to be able to have a email client in the browser and do online banking, or edit Wikipedia articles in a rich html editor, or play games, or watch videos and share and comment them or even do video editing, ... all in the browser.
IMO they largely just want to do all these things in an open core VM, without risking installing malware on the ridiculously insecure proprietary OS most of them use if/when they use desktop computers.
No, then it is not.
Or do you mean in academical sense of functionaliy per byte? How useful is that?