iPhone < 5:
@media screen and (device-aspect-ratio: 2/3) {}
iPhone 5:
@media screen and (device-aspect-ratio: 40/71) {}
iPhone 6:
@media screen and (device-aspect-ratio: 667/375) {}
iPhone 6 Plus:
@media screen and (device-aspect-ratio: 16/9) {}
iPad:
@media screen and (device-aspect-ratio: 3/4) {}