The img@srcset does not afford developers the ability to define the breakpoints for images as either minimum values (mobile first) or maximum values (desktop first) to match the media queries used in their design.
See: “Secret src” http://adactio.com/journal/5474/
Commenting per the HTML WG’s request:
This bug would be addressed by the proposed `picture` extension specification: See: http://picture.responsiveimages.org/#relationship-to-srcset
Further discussion is available in the WHATWG clone of this bug: https://www.w3.org/Bugs/Public/show_bug.cgi?id=20212
The two alternatives are mathematically equivalent, and because of this everyone agrees that it's *possible* to express either a "mobile first" or "desktop first" set of breakpoints. Given this, I think the complexity costs of adding syntax to improve the "desktop first" case outweigh the benefits.