Specifically, a tool called GroupXIV based on Leaflet. https://github.com/whitequark/groupXIV
The viewer is really cool. I wonder if it'd be possible to modify it to display a folder of PNGs in a nice streamable way...
(It looks like it only supports slicing a single png, whereas we usually have tens of thousands of pngs. I've been thinking of ways of letting people view lots of ML images better than e.g. how tensorboard does it.)