There's a media query available (with prefixes, right now) for pixel density – a retina MBP reports it as 2 whereas a standard one reports it as 1. So you can have a media query for a pixel density >= 2, etc.
Right, which is what concerns me. So is this the expectation?
<img src="small.jpg" data-interchange="[normal.jpg, (only screen)], [medium.jpg, only screen and (min--moz-device-pixel-ratio: 2),
only screen and (-o-min-device-pixel-ratio: 2/1),
only screen and (-webkit-min-device-pixel-ratio: 2),
only screen and (min-device-pixel-ratio: 2),
only screen and (max-width: 749px)]">
Right, which is what concerns me. So is this the expectation?