[gerrit]

The user experience when viewing webpages is improved by actually showing a lower-resolution image while the crisp high-resolution image is being downloaded (however slowly), similar to progressive JPEGs.

Depending on the data contract, the user experience when viewing their mobile phone bills might be affected negatively though.

[downx3]

Would it be better to sniff the header?

Accept: RETINA

Something like that, at least then you could turn it off in the browser.

[downx3]

Might be just as well to sniff the user agent.