Great little write up on Site-Point on how to create a mobile-friendly off-screen menu using ONLY CSS:

https://www.sitepoint.com/pure-css-off-screen-navigation-menu/ Read more about Pure CSS Off-Screen Menu

Here's a great cheatsheet for pinpointing the exact breakpoints for the various flavors if iPhones and iPads.

http://stephen.io/mediaqueries/ Read more about Various Breakpoints for Devices