Excellent, sorry I was not clear on that; this is looking good! I would like to re-iterate that this solution is another which puts design properties into mark-up directly, and just like old <picture> and srcset, this means that when it's time to re-design a site an author is going to have to trawl through all <picture> elements throughout the site to adjust them to fit the new design.
This is to my mind a major problem which stops any such solution from being a general-purpose solution. I'd be ok for one-off special uses, but I can't write website's that I know to be future un-friendly - that's just storing up problems for the future. -Matt [re-posted to the group, I'd hit reply and not reply-all, sorry!]