CSS pixel

visual angle of about 0.0213 degrees

A CSS pixel is the canonical unit of measure for all lengths and measurements in CSS. This unit is density-independent, and distinct from actual hardware pixels present in a display. User agents and operating systems should ensure that a CSS pixel is set as closely as possible to the CSS Values and Units Module Level 3 reference pixel (opens in a new tab) [css3-values (opens in a new tab)], which takes into account the physical dimensions of the display and the assumed viewing distance (factors that cannot be determined by content authors).

Source: WCAG 2.2 definition of “CSS pixel” (opens in a new tab) (W3C)