HTML+CSS is designed to make things as device-independent as possible. If the definition of "width: 100px" were to be vastly different on different devices this goal would be unobtainable. As long as the definition of px is some integer multiple of device pixels I think it's a useful compromise. If you really need precision at the level of device pixels you should be writing a native app.
It's a useful compromise for the OS to make, but does not belong in CSS. If I'm running on a non-native resolution, I expect the image containers to scale on websites to fit the images, which they won't if the browser overrides my 1.4 pixel pixels.
Whatever did happen to screen pixels, anyway? Did non-native resolutions die with CRT screens? It's nice to have a unit for 0.27 mm-ish, but I'd really have more use for a pixel.
