Web developer Le Roux Bodenstein:
I propose a fifth option [to handle responsive images.]
Use a progressive image format and HTTP range requests. Ideally the image metadata at the start of the file would include some hints about how many bytes to download to get an exact image size. The browser can then download the smallest size equal to or greater than the dimensions it needs based on the layout’s width as specified in CSS. If the user zooms or resizes the window, the browser can request more bytes as needed.
One image, one file, one URL, one resource.
Ballsy. Yet even as the author points out, there are significant legacy browser compatibility issues.