CSS: How Large is a CSS Pixel?
CSS: How Large is a CSS Pixel?
px
, in practice, is 1/16 of default font size of a device.
px
is defined as reference pixel, which is a visual angle of 0.0213 degrees. This means, the larger the screen, the larger is 1px
.
px
originally was the same size as a physical pixel of desktop displays in the 1990s.

CSS Spec on What is a Pixel

