Not that it matters much, but Apple didn't actually have to redefine a CSS pixel. They foresaw this when writing the CSS specification:

    If the pixel density of the output device is very
    different from that of a typical computer display,
    the user agent should rescale pixel values. It is
    recommended that the reference pixel be the visual
    angle of one pixel on a device with a pixel density
    of 96dpi and a distance from the reader of an arm's
    length.