px

The CSS pixel is the unit the rest of the absolute family comes down to. An inch is defined as exactly 96 of them, and the point, pica, centimeter, millimeter and quarter-millimeter are each a fixed fraction of that inch, so every absolute length is a fixed number of CSS pixels. Nothing in this chain touches the screen in front of you. The specification ties the pixel to a reference pixel, the angle that one pixel of a 96 dpi screen covers in the eye at arm’s length, and the idea behind that definition is that one px should look about the same size on a phone, a laptop and a television.

Lengths in a stylesheet are counted in CSS pixels. The hardware shows up only at the last step, where the browser maps each CSS pixel onto as many device dots as it needs: a 2×2 block on a display with a device pixel ratio of 2, half a dot across at a ratio of 0.5. A box of 100px measured 100 CSS pixels at every ratio from 0.5 to 3 in Chromium, so a change in the ratio changes how many dots a px covers while every px value in your stylesheet stays the same.

A length in px knows nothing about the reader’s text settings. Ask for 16px and you get 16px, whatever default font size the browser has been told to use: in Chromium with the default raised to 24px, 1rem became 24px while 1px, 1pt and 1in stayed as they were. Keep it for things that should hold still: hairlines, shadows, the box around an icon with a fixed bitmap behind it.