This proposal adds new elements and attribute to [HTML5] to enable different sources of images based on browser and display characteristics. The proposal addresses multiple use cases such as images used in responsive web designs and different images needed for high density displays.
This proposal allows content authors to provide user agents with the information they need to select the best image source. The current img element only allows for a single source of an image, but there are numerous use cases where document authors need to define different image sources depending on the factors such as the design, size resolution, and display density.
The best image source may be an image sized appropriately for the display size or pixel density. Or the best image source may be a different version of an image that has been modified by the author to be suitable for a particular use (see: art direction use case).
» w3.org