The distinction is also in CSS specs[0] since version 2.1: “The reference pixel is the visual angle of one pixel on a device with a pixel density of 96dpi and a

The distinction is also in CSS specs[0] since version 2.1: “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. <…> 1px thus corresponds to about 0.26 mm (1/96 inch).” [0] http://www.w3.org/TR/CSS21/syndata.html#length-units http://www.w3.org/TR/CSS21/syndata.html#length-units See also http://inamidst.com/stuff/notes/csspx http://inamidst.com/stuff/notes/csspx (been featured on HN some time ago)