 CSS pixel

A CSS pixel, written px, is a unit of length in CSS layout. Its reference definition concerns a visual angle, while the specification fixes the ratios between CSS absolute length units:within the CSS unit system. The browser can anchor these units to physical measurements or to a reference pixel appropriate to the output medium. Consequently, a CSS pixel does not necessarily equal one physical display pixel.^[ref]

For example, a box with width: 100px has a declared width of 100 CSS pixels. On a display mapping two device pixels to each CSS pixel along each axis, that width spans 200 device pixels before any additional transforms. A square of side 100 CSS pixels spans a $200\times 200$ device-pixel region in that simple mapping. The device pixel ratio connects these two coordinate scales.

The fixed ratio 96px = 1in does not measure a monitor's physical pixel density. A browser's on-screen 1in can differ from a measured physical inch when the reference-pixel anchor is used. Page zoom can also change the physical size of the layout's CSS pixels, even though the declared number 100px remains the same.^[ref]