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Echoing what others said. picture and srcset aren’t beautiful (neither is much of HTML or the web stack). And verbosity is a problem.
A new image format will take ages. And content negotiation doesn’t work where there is no server (in an increasing number of environments such as epub, or an HTML front end to assets in kiosk etc).
I had sympathy with the idea that art direction is a CSS matter. But debating that is fruitless anyway; by the time the browser has the CSS, it’s already started to download the image that has an src in the markup.
The only way round this now is to use an empty div in the markup and use media queries to drag in a different background image using WebKit’s img-set http://blog.cloudfour.com/safari-6-and-chrome-21-add-image-set-to-support-retina-images/. That would lead to lots of content images being removed from content, added to the CSS layer so that they are visually there but have no alt text.