Comment by jillesvangurp

CSS got it right by making pixels a relative unit. Meters are absolute. You cannot express pixels in meters. Because they are relative units.

If you have a high resolution screen the a CSS pixel is typically be 4 actual display pixels (2x2) instead of just 1. And if you change the zoom level, the amount of display pixels might actually change in fractional ways. The unit only makes sense in relation to what's around it. If you render vector graphics or fonts, pixels are used as relative units. On a high resolution screen it will actually use those extra display pixels.

If you want to show something that's exactly 5cm on a laptop or phone screen, you need to know the dimensions of the screen and figure out how many pixels you need per cm to scale things correctly. Css has some absolute units but they only work as expected for print media typically.