title: Retina - Correlation between device pixel ratio and size of image?
body: <p>I don't quite understand what the <code>window.devicePixelRatio</code> value is, and how it dictates what size image (2x, 3x, etc) I need for that device.</p>
  
  <p>For instance, on an iMac 5K Retina (Late 2015), I'd expect the pixel ratio to be at least 3 or so, but it's actually 2, the same as an iPad Air and iPhone 6s. Does that mean it prefers a 2x bitmap? 3x?</p>