[blacha]

looks to be using open sea dragon [1] This is just a tiled map so any web mapping software would also work OpenLayers, Leaflet, Maplibre etc.

[1] https://openseadragon.github.io/

[_joel]

Interesting, I've used leaflet a lot in the past (love it!) for GIS but never thought about tiling a more generic image to it. Good call.