screen.width:

screen.height:

devicePixelRatio:

This is the max-width div.

blue: <= 320
yellow: >= 321 && <= 479
green: >= 480

This is the max-height div

yellow: <= 320
red: >= 321 && <= 480
blue: >= 481

This is the device-width div.

green: <= 320
blue: >= 321 && <= 480
yellow: >=481

This is the device-height div

green: <= 320
blue: >= 321 && <= 480
yellow: >= 481