How do you turn a fictional map into a one page website?

Let’s say you have a .png of some fantasy world map and want to turn it into an interactive web experience. I know that you could use L.imageoverlay with Leaflet and then work with markers (like https://www.techtrail.net/creating-an-interactive-map-with-leaflet-js/).

But what would you use to make something like this: https://bitsummit.org/gaiden-map/? Where separate areas of your .png are different selectable menu pop ups?

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