The three-year journey to multitasking and resolution-independence in iOS 9
arstechnica.com/apple/2015/06/the-three-year-journey-to-multitasking-and-resolution-independence-in-ios-9
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
Auto Layout, size classes, and the other features developers should be using now.