A word of warning...

One strong reason to use srcset is to reduce the downloading of large images on mobile devices.

A major issue with this solution is that it downloads the image defined in src as well as an additional image defined in srcset. This effectively increases the amount of downloading a mobile device has to do.

From the project's demo:

Because, the w attribute is defined by whatwg as maximum viewport width you cannot simply put the smaller file into src and larger files into srcset.

On the plus side...

If you're using this polyfill to provide HiDPI users on fast connections with higher quality images this solution is a good solution as long as you're aware that two images will be downloaded.