He writes it directly in his article:
> My monitor’s native resolution is [w × h device pixels]. But I find the fonts slightly too small at that resolution – given how far I sit from the screen. [So I use DPI scale factor R, what] translates to an effective screen resolution of [w/R × h/R virtual (CSS) pixels].
(In his case it is 1080 × 1920 / 1.5 = 720 × 1280). Measuring screen dimensions in physical display points, like
var dppx = window.devicePixelRatio;
var screenWidth = screen.width * dppx;
var screenHeight = screen.height * dppx;
The moment he decides to get another second vertical monitor and put both just a bit farther he will be back on the square one.
You may have super tiny device with super high density display close to your nose, look at a screen on opposite side of your room that can have same density (what would be a bit wasting resources, but anyway) or look at super large billboard screen across the road.
Until the page will know exactly: - how far is the screen from eye of its observer, - observer's eyesight conditions, - and observer's perceived visual size preferences, there will be no way to solve virtual pixel problem the way that will make everyone happy.
(And that's quite OK, I think.)