I think the only reasonable approach applications can take is to layout everything in physical sizes, and to be largely unassuming about the display device (i.e

I think the only reasonable approach applications can take is to layout everything in physical sizes, and to be largely unassuming about the display device (i.e. assume a standard (desktop monitor?) viewing distance). In consequence a sensible default font size would be something like 14pt, etc. If the results aren't suitable (maybe a desktop monitor is viewed at a larger distance than a phone screen, maybe user is visually impaired, etc) then the OS can still offer to globally or locally configure the screen with a fake dpi.