I’ve been playing with QGIS, and it’s an awesomely powerful piece of software for map making an analyzing geographical data.
It has the ability to show HTML map tips when you hover over certain features, but with some restrictions.
The containing box (which you can’t style, AFAIK) has
- white background
- 1px border in grey (#120909),
- 8px padding top & left and 12px padding right and bottom
The maximum size of the box is limited to half the size of the graphics area, which means scroll bars appear within the map tip if the content plus the padding and border would make it bigger than that.