WordPress 4.4 shipped this week with a really interesting approach to responsive images.
Joe McGill, who led the project, has an overview of the changes:
WordPress automatically creates several sizes of each image uploaded to the media library. By including the available sizes of an image into a srcset attribute, browsers can now choose to download the most appropriate size and ignore the others—potentially saving bandwidth and speeding up page load times in the process.
That was the goal. Here's an example of the actual code for an image in WordPress 4.4: