UHD is not the only HiDPI/retina resolution. My 15" MacBook Pro has a hardware pixel resolution of 2880x1800 but reports "CSS pixels" of 1440x900. The report be

UHD is not the only HiDPI/retina resolution. My 15" MacBook Pro has a hardware pixel resolution of 2880x1800 but reports "CSS pixels" of 1440x900. The report being from Firefox, all the iOS devices with retina displays aren't represented at all. I don't know how ubiquitous HiDPI displays are on Android devices but my hunch is most smartphones sold in the U.S. and Europe have them.