The numbers on the top are the calculated resolutions for the image:
number in green is the image selected. While number in gray is the next lower and number in red is the next higher candidate.
The image on the left side has the data-optimumx attribute attached, while the same image on the right side is always loaded with the full density.
As you will see with this demo. Some images do gain significant quality with retina, but some others don't. The standard HTML responsive image markup has no way to tell the browser about does differences, therefore the browser often can't make educated decisions. The optimumx plugin solves this by limiting the source canidates to reasonable ones.
