It would be nice if WebKit supported the <img src="…" lowsrc="…"> HTML construct. That way one could use it to deliver 72dpi images first and then higher res images for high DPI sites, without any funky CSS media queries. See linked site for an example of it, and an example with a larger main image here: http://www.scit.wlv.ac.uk/encyc/lowsrc.html
I don't think this is a good approach for 72 DPI vs. higher DPI.
Perhaps not optimal, but it is a solution that's backwards-compatible with IE and NS. But the High DPI case is not the reason I requested it. If a website has images (that take a long time to load, for example NASA has lots of TIFF files on it's site), they could load in the background while a small progressive JPEG is loaded as the lowsrc, giving all of fast very lossy display -> lossy display -> slow lossless display. Perhaps not NASA themselves, but their images could be linked to from a third-party site in this way. I'm sure there are many other real-world-usage examples that would be implemented if the attribute were better supported. And given that it never reached any HTML specs (to my knowledge), it would be a quirks-mode only feature.
URL is dead.
This can be achieved via modern CSS and <picture> tag. Modern CSS can enable you to request high-dpi images and also have "progressive" enhancement to reduce data bandwidth costs. I think this is achieved via various other means, so this would "RESOLVED WONTFIX" at the moment. Thanks!