I have no idea what ACF is, but I probably don't need to know.
There is two ways to make responsive images (CSS-only) work. Either
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<img class="WHATEVER" src="URL" alt="ALTTEXT" width="WIDTH" height="HEIGHT">
or
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<img class="WHATEVER" src="URL" alt="ALTTEXT">
Both work fine with the responsive CSS solution and scale properly. The first one is preferrable because of the page rendering perfomance improvement.
If I understand you correctly, all that is possible with CKE without hacking it too much is
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<img class="WHATEVER" src="URL" alt="ALTTEXT" style="width:WIDTH">
which is pretty bad.
That would mean we'd have to “overwrite” the width CSS attribute (not HTML attribute) with “auto”, which could have side effects, and would lose the better rendering due to the lack of width/height HTML attributes. Also, we'd have to find a selector which provides more specifity than the inline style, which most likely means !important.
Trying to think outside the box here – do we
have to use the media library button in CKE or could we add our own media library button outside of CKE which still inserts code into CKE?
YL