<img srcset="http://placehold.it/240 2x" onload="alert(this.naturalWidth)">
Per spec it should alert 120. WebKit alerts 240. Gecko and Blink match the spec, I don't know about Edge.
I would recommend waiting with fixing this bug until .currentSrc is implemented, since exploiting this bug can be a poor man's .currentSrc; see http://stackoverflow.com/questions/33231426/get-the-currentsrc-of-responsive-image-on-safari/33278821