The px unit in css is a bit misleading, the measurement doesn't always correspond to the size of the device pixels. From the spec[1]:
"For a CSS device, these dimensions are either anchored by relating the physical units to their physical measurements, or by relating the pixel unit to the reference pixel. For print media and similar high-resolution devices, the anchor unit should be one of the standard physical units (inches, centimeters, etc). For lower-resolution devices, and devices with unusual viewing distances, it is recommended instead that the anchor unit be the pixel unit. For such devices it is recommended that the pixel unit refer to the whole number of device pixels that best approximates the reference pixel."
Comments
The px unit in css is a bit misleading, the measurement doesn't always correspond to the size of the device pixels. From the spec[1]:
"For a CSS device, these dimensions are either anchored by relating the physical units to their physical measurements, or by relating the pixel unit to the reference pixel. For print media and similar high-resolution devices, the anchor unit should be one of the standard physical units (inches, centimeters, etc). For lower-resolution devices, and devices with unusual viewing distances, it is recommended instead that the anchor unit be the pixel unit. For such devices it is recommended that the pixel unit refer to the whole number of device pixels that best approximates the reference pixel."
[1] http://www.w3.org/TR/css3-values/#absolute-lengths