To call a CSS pixel not an actual pixel but a measurement of an angle at a reading distance is... technically correct... but isn't really representative of how it's actually rendered. On a non Hi-DPI display at 100% scaling in the browser and OS, a px is a device pixel. An css inch is considered 96px, with the assumption that a CSS inch is an imperial inch on a 96 DPI display.

Yes, it's a bit fuzzy now with modern displays (especially when display scaling is not at a whole number haha) but it just kinda feels like searching for something to complain about. If your browser and OS doesn't get in the way you can get exact measurement. This also applies to almost any UI framework that has some integration with the OS.