I was wondering why IMG has been left out. Video and Sound have a source tag which I think IMG should have as well so that I as a web developer can have different image resources depending on things like the screen size or what the browser supports
There's some ongoing talk about it. You may find some info here:
And probably some other places on the web too ;-)
Why <img> hasn't got that is first and foremost just legacy. But now that we want something like it, it's harder to say how.
(and with a HTTP-based solution, having it all in one progressive image, allowing to stop downloading that one when you don't want more, then img tag wouldn't have to be changed to have sources at all).
This bug was cloned to create bug 17848 as part of operation convergence.
Mass move to "HTML WG"
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Status: Partially Accepted
Change Description: No spec change.
Rationale: The HTML WG is actively pursuing work in this area, in the
form of the <picture> and srcset="" extension specifications. I
encourage the OP to contribute to those efforts.