The Nokia Lumia Icon has a screen resolution of 1080x1920, but CSS reports a resolution of 403x659 (portrait) and 716x346 (landscape). That's rather annoying. Do something, logical, like 320x480/480x320, half-resolution (540x960/960x540), or full-resolution. Don't use some random, arbitrary, numbers, though. That just makes thing difficult for developers.