>If you spurn px entirely (as some people appear to do) how are you going to get this magic length?
You use a high-resolution screen such that 1 pixel is too small to make a line, and on lines thick enough to be visible the effects of antialiasing are subtle enough to ignore.
Well there's not really any other way to ensure crisp rendering. A user with a low resolution screen, even if 1px equates to an actual pixel on their device, might have web pages zoomed in by 15% or out by 10%, breaking all attempts to avoid antialiasing.
You use a high-resolution screen such that 1 pixel is too small to make a line, and on lines thick enough to be visible the effects of antialiasing are subtle enough to ignore.