Attribute srcset not allowed on element img at this point.

Sample document:
http://en.wikipedia.org/wiki/User:Gadget850/srcset

When validating an English Wikipedia page, the error "Attribute srcset not
allowed on element img at this point" is displayed. This is due to the
srcset attribute being used to generate alternate images.

Srcset is not in the live HTML5 specification:
http://dev.w3.org/html5/spec-author-view/the-img-element.html#the-img-element

But is in the living standard:
http://www.whatwg.org/specs/web-apps/current-work/multipage/embedded-content-1.html#attr-img-srcset