When images are included on a page, the srcset attribute is included in the <img> tag. This is not allowed in HTML5.
http://en.wikipedia.org/wiki/User:Gadget850/srcset
http://validator.w3.org/check?uri=http%3A%2F%2Fen.wikipedia.org%2Fwiki%2FUser%3AGadget850%2Fsrcset
http://dev.w3.org/html5/spec-author-view/the-img-element.html#the-img-element
Version: 1.21.x
Severity: minor