I have a notebook with a 132 DPI display. Due to this normal web pages are very small, and the content must be zoomed so that I get a sufficient font size. Regarding this, I would say that settting the font size for example to 12 dpi (or x EM) will help. My desktop is configured in order to get a nice size for text output. If I attach a second display with a lower DPI, all textes showed on the external screen will be to big, modern systems don't look for the DPI setting for each display, this is wrong.
The problem about you write about is primary the viewing distance / output media and secondary the user preferences.
If the systems will primary look at the display DPI, the type of output device (computer display, television, tablet, book (viewing display / distance) and finally at the user prferences (small, medium big size) the problems can be solved in an easy way.
UNfortunatelly most developper don't think about this.