alt Hacker News> Accurate DPI information is insufficient as users may want to scale differently anyways, either due to preference, higher viewing distance, or disability.
Which is fine. There's already a standardized property in XSETTINGS to use on X11 to advertise the user's scaling preference. For Wayland they decided to include this into the protocol, so it can be per-output and/or per-window (though the per-window fractional scaling stuff is an optional extension, sigh).
There's no reason why we couldn't do something similarly on X11, via xrandr output properties and X window properties. But it's more fun to abandon things and invent new ones than fix the things you have, so here we are.
The issue is that if you want DPI virtualization, and all of the desktops do want that, you need to be able to translate all of the coordinate spaces everywhere. If you try to integrate this into X11, you run into a myriad of different problems that are just not that easy to solve. It was direct inspiration for some of the design elements in Wayland.
The same folks who are working on Wayland today did a lot of work to get X.org to where it is now. They could do more, but the writing was on the wall.