HTML Does Not Need Retina Images
Smart people are spending a massive amount of time trying to solve a problem that doesn’t exist.
Trying to retrofit HTML to allow for things like quadruple-resolution images for HiDPI monitors, and lower quality images for slower connections (ie. mobile).
Why not just upload quadruple-resolution Adam7 interlaced PNGs and, if the browser does not support HiDPI, the browser can simply stop downloading the image after the fifth pass.
On mobile, or any other connection where bandwidth is more of a concern than quality, it could stop after an even earlier pass.
Either way, making most efficient use of bandwidth must be the job of the user agent more then the content itself.
There must be a better solution than adding new (redundant) tags to HTML.