All hands on deck!
Older versions of Picturefill can net you broken images in both Microsoft Edge and Webkit Nightly. There’s already an issue logged for the problem.
Why is that an issue?
The community is afraid that browser vendors like Microsoft will renege on their pledge to implement the Responsive Images spec because the impact of having no images shown is too big.
(And update picturefill to version 2.3.1 or newer.)