From the CSS specification: px: pixel units — 1px is equal to 0.75pt. [...] The reference pixel is the visual angle of one pixel on a device with a pixel den

From the CSS specification: px: pixel units — 1px is equal to 0.75pt. [...] The reference pixel is 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. For a nominal arm's length of 28 inches, the visual angle is therefore about 0.0213 degrees. For reading at arm's length, 1px thus corresponds to about 0.26 mm (1/96 inch). http://www.w3.org/TR/CSS21/syndata.html#length-units http://www.w3.org/TR/CSS21/syndata.html#length-units