Here is an incomplete list of some devices and ratios:
1.3x Google Nexus 7 1.5x Google Nexus S Samsung Galaxy S II HTC Desire HTC Incredible S HTC Velocity HTC Sensation 2x iPhone 4 iPhone 4S iPhone 5 iPhone 6 iPad (3rd generation) iPad 4 All Retina displays Mac Google Galaxy Nexus Google Nexus 4 Google Nexus 10 Samsung Galaxy S III Samsung Galaxy Note II Sony Xperia S HTC One X 2.6x Nokia Lumia 930 3x Xperia Z Xiaomi 2 Galaxy Note HTC One (M7) HTC Butterfly
Here is an answer from the spec:
The srcset attribute is used with the src attribute, which gives the URL of the image to use for legacy user agents that do not support the srcset attribute. This leads to a question of which image to provide in the src attribute.
The answer that results in the least duplication is to provide the image suitable for an infinite width and infinite height viewport with a pixel density of 1 CSS pixel per device pixel.
References