[whatwg] add html-attribute for "responsive images"

[Bjartur Thorlacius]

> How exactly can I create a dynamic CMS website today with responsive images?
> I could manage my CSS file with my CMS which most of them don't support. I could minify the CSS on the fly but in reality and from performance view it's recommended to serve a static css file from a cookieless domain (which is another than where my CMS is located). If I would use a CDN additionally how should I change images on the fly? And, a client wants to change an image in a WSIWYG editor not in a raw css file. We won't either as he can destroy the entire stylesheet. I hope you now understand better what I mean.
But he won't edit "the entire stylesheet"; he'll create a new one.