Another deep dive into the application of the new picture element. Tab Atkins even took the time to comment on the post and point out some issues with the <picture> and art direction.
Most new phones and tablets are high resolution i.e. they have more than one physical device pixel for every CSS pixel. If we have an image that isn’t going to vary too much or at all in width we can give hi-res screens a hi-res image, and low-res screens a low-res image.An excerpt from Responsive images | No strong beliefs