https://html.spec.whatwg.org/multipage/embedded-content.html#the-img-element currently requires:

Thesrc attribute must be present, and must contain avalid non-empty URL potentially surrounded by spaces referencing a non-interactive, optionally animated, image resource that is neither paged nor scripted.

and

Thesrcset attribute may also be present, and is asrcset attribute.

In other words, a validator should complain about animg element with asrcset attribute but nosrc, andhttps://validator.nu/ does.

However, it's not clear why it's important to require putting one source in thesrc attribute, instead of putting them all insrcset. The processing model deals withsrc being missing:

• Inupdate the source set,default source will remain the empty string
• Increate a source set, ifdefault source is the empty string, obviously there's nothing to add tosource set

In other words, omittingsrc works just fine in practice.

Whensrcset was new it made sense to require asrc, but nowsrcset has been supported everywhere for many years:
https://developer.mozilla.org/en-US/docs/Web/API/HTMLImageElement/srcset#browser_compatibility

Is it time to makesrc optional and not require sending unused markup like this?