WordPress 4.4 adds native srcset support. Core currently only look to the width descriptor so browsers can choose the right image width based on need. The spec also supports pixel density, e.g. 1x, 1.5x, 2x, etc, so we should be able to use this, possibly deprecating device-px.js (within Jetpack at least) within a few versions.

Initial read is that you can't use both the w and x descriptors for the same image set, though.

Marking as 3.9 for now.