Absolute measurements (which no longer exist in CSS) are a sane approach to support different resolution displays that are all used at the same viewing distance.
Using absolute measurements to size things on a web page that is then viewed on a TV (viewing distance in the 5-15ft range), a tablet (viewing distance in the 1-2ft range), and an eyeglass HUD (viewing distance in the 1-3in range) would be a disaster.
The fact that people _were_ using inches and millimeters on the web and expecting them to somehow work across all these devices is why they're all now defined in terms of CSS reference pixels...
Using absolute measurements to size things on a web page that is then viewed on a TV (viewing distance in the 5-15ft range), a tablet (viewing distance in the 1-2ft range), and an eyeglass HUD (viewing distance in the 1-3in range) would be a disaster.
The fact that people _were_ using inches and millimeters on the web and expecting them to somehow work across all these devices is why they're all now defined in terms of CSS reference pixels...