Enable a responsive images solution for the "resolution switching"/"variable width images" use case, by letting authors hint the browser regarding eventual layout dimensions, and provide multiple resources in multiple dimensions. As a first stage, these features would be implemented only on the img element.
The "resolution switching" use case is a major use-case of the responsive images problem, and solving it would enable authors huge savings when designing Responsive/fluid Web sites.
The sizes and extended srcset attributes, as defined in the picture specification, resolve that use case in an efficient and elegant manner.
That feature is a subset of the overall picture specification, which Firefox are also working on.
There is no official response from WebKit regarding support for this feature, but WebKit community members have expressed favorable opinions about the feature.
There is no official response from IE regarding their intentions about this feature.