X can easily support per-display DPI -- clients just have to eschew the legacy global DPI setting. The Xrandr extension knows the dimensions of each display in both pixels and millimeters from its EDID. Clients interested in making adjustments based on per-display DPI could simply use those values obtained from Xrandr. These values can be wrong, of course -- monitors have been known to lie in their EDID reports -- bu…
xrandr is not sufficient for this. One needs a way for users to configure the per monitor DPI. And have the configuration available to X clients. Currently the only cross DE mechanism for this is Xft.dpi and its global, not per monitor.
xrandr --setmonitor has entered the chat