Retina display support

96 CSS pixels per inch is normal (or at least nominal) for a desktop computer screen. A handheld device is typically going to have a substantially different viewing distance, and as a consequence it isn't at all natural to display the same content at the same number of CSS pixels per inch. A CSS pixel should (roughly) encompass the same angle, not the same physical size.

As a result, 2 x 2 device pixels per CSS pixel is just about ideal for a 192 DPI desktop display. Ideal as in minimizes aliasing for pixel oriented page designs, which are commonplace. If you have a handheld device, the DPI should be higher because the viewing distance is going to be less. 384 or even 400 dpi sounds about right for 2 x 2 scaling (2 dppx) on a device like that. 4x4 scaling someday (~768 DPI handheld, 384 DPI desktop) sounds well within the retina limit to me, at least if you want to avoid eyestrain.