The most important thing to take away from the original is that the css unit "px" has no relationship to the actual size of a pixel on the screen

Did you read this article?

Did you make it to this part?

    I’ll just finish by going through a few of the explicit
    claims made in that article and correcting them:

    [...]

    2. The first sentence: “The “px” unit in CSS doesn’t really
    have anything to do with screen pixels, despite the poorly
    chosen name.” – this is obviously complete nonsense. The
    standards authors went out of their way to give browser
    authors a way to match the “px” unit up to the real-world
    device pixel

> and all the physical units (inch, cm, pt) are defined in terms of the csspixel.

Not for print and other high-res devices (like a Retina display). Did you read this excerpt quoted from the CSS spec?

    For print media and similar high-resolution devices, the
    anchor unit should be one of the standard physical units
    (inches, centimeters, etc). For lower-resolution devices,
    and devices with unusual viewing distances, it is
    recommended instead that the anchor unit be the pixel unit.