For now that is down to the browser to determine which file to load. From a design perspective, it makes sense to have the HQ image load full screen on click.
Yes, I get that. The problem is there isnt much for the browser to base its heuristics on. Would you send HEAD request for all the listed assets to at least get file sizes? This will be slower and might be even more traffic than just grabbing first one.
Requiring listed size/quality for all the sources might not even be feasible without further automation on the server side, not to mention potential content mangling/"optimizing" proxies/appliances.
HEAD requests seem like the only option aside from inferring size from suggested resolution and format. Imagine trusting the author to include the resource size.
If I could add to the spec, I would put something for lofi/hifi. Then of course that is all subjective and developers will eventually use it in problematic ways.
There's already enough gaming around pagespeed scores. If such an incentive is added, the spec needs to be formulated in an ergonomic way to avoid mal-optimizations.
