I agree that most websites are built inaccessibly and respond poorly (if at all!) to user’s font-size preferences. What I’m advocating for here is to use rem for font-sizes and em or other font-size-relative units for content-related spacing. Using relative units for spacing would fix the line-height issue and the horrendous overflow problems seen in Gmail and other apps.

By *completely* eliminating px units from our repertoire, we could avoid these issues and focus on designing for dynamic content. Ideally, a site displayed on a Jumbotron with a base font-size of 2000px would look as good as it does on a small phone with a base font-size of 14px. People are going to view your content on everything from 65" TVs to their smartwatch and everything in between (half-circle car displays, smart mirrors, whatever). Sometimes they’ll be standing close to the display and sometimes they’ll be across the room (or ballpark), so the text and layout should be able to be zoomed and still readable/beautiful.