The question you should be asking is, why is the default CSS in your browser making unstyled websites uncomfortable to read? The browser could provide sane defaults for margins etc. It just chooses not to - probably because everything it renders is going to be styled anyway, so why bother?

Ironically, the "reading mode" thing that is becoming popular is essentially that - stripping the page of most of its custom styling, and then applying some sensible default style. It's like we went full circle, all the way back to pre-CSS times.