Earlier quoted context omitted.
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/CSS2…
Thanks! I didn't know that a CSS px wasn't the same as a screen pixel.
By default, something declared as 16px is actually rendered as 32px (but sharper).