Sorry, I'm missing something. How come a regular SVG image, without any JS wouldn't work on retina displays? They're already vector images, and would rasterize on a machine that knows its own DPI.
Vector SVG graphics absolutely do work great on retina displays. This is for serving raster retina images, and the JavaScript enables you to request only the resolution appropriate for each client.
I'm not sure, although the implementation of SVG-as-`img` is still fairly buggy for me as far as dynamic resizing goes. This technique doesn't help with that, I'm just pointing out that the support isn't really all the way there.