To warm up this topic: I still believe this is necessary, despite all the arguments that the CommonMark text should not concern itself with how it is being presented / rendered later. While I agree that specifying exact sizes or HTML attributes into CommonMark is not reasonable, I think that at least a way to express “this image should be scaled to X% of the text width” is IMO necessary to make images in CommonMark even remotely usable. Otherwise the rendering engine has an impossible task: It cannot automatically decide whether an image should be up/down-scaled or included 1:1 just by looking at the image and the output format.