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In the "Content attributes" subsection for <source> at http://www.whatwg.org/specs/web-apps/current-work/multipage/edits.html#the-source-element-when-used-with-the-picture-element there are no short descriptions for any of the attributes for <source>. For each it should have, e.g., "srcset — Images to use in different situations (e.g. high-resolution displays, small monitors, etc)". Similarly, the "Content attributes" subsection for <img> at http://www.whatwg.org/specs/web-apps/current-work/multipage/edits.html#the-img-element lacks a short description for the the "sizes" attribute.
This looks to be fixed in the latest spec; please correct me if I'm wrong. Presumably zcorpan did it during the course of https://github.com/whatwg/html/issues/52
Yep, https://github.com/whatwg/html/commit/dfe18acf2b4834be5b63cf6bebb97245a1b2a910