[beanlog]

It'd be more accurate and illuminating to say that a px is an angle.

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). - https://www.w3.org/TR/css-values-4/#absolute-lengths

This explains why 1px is small on phones and large on televisions; the physical size varies by expected reader distance. The angle tries to stay constant, though in practice it gets rounded to the nearest multiple of the physical pixel size.