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http://www.w3.org/TR/html5/embedded-content-0.html#the-img-element does not list srcset as an allowed attribute. However, it is referenced in relation to 'complete ("Both the src attribute and the srcset attribute are omitted.", "The srcset attribute is omitted and the src attribute's value is the empty string."). It seems inconsistent to refer to an attribute that is not defined in this version of the standard.
I'll take this; I've already got the relevant START and END directives in the secretes branch, so it should be an easy merge.
s/secretes/srcset/ (DYAC!)
Fixed in 5.1 in 1b4d8c1 and in 5.0 in c4d03b4. (Look, ma, no boilerplate!)