On: In CSS, “px” is not an angular measurement and it ...

[cynwoody]

>but OS X treats it as 72dpi

I just added a 960px <hr> to this web page. On my monitor, attached to a MBP running OS 10.6, it measures about 9.5 inches (by holding up a ruler to the screen). That would indicate OS X treats my screen as about 100 dpi.

The monitor is 30" diagonal, 2560x1600. So, calculating the dpi:

    irb(main):010:0* ratio = 1600/2560.0
    => 0.625
    irb(main):011:0> width = 30.0/Math.sqrt(1 + ratio*ratio)
    => 25.4399491201526
    irb(main):012:0> dpi = 2560.0/width
    => 100.62913207527

One of these days, I'm going to get around to buying a new Retina MBP. Then, I'll try that exercise again.

[jahewson → cynwoody]

Yep, that's because it's 960px in CSS pixels, which are at 96dpi. Your monitor also has it's own physical dpi so you can't just measure the length of a line on the screen to infer the OS's dpi.