theoreticalmal's comment seems to imply that taxes on unrealized gains are not a thing, to which I was replying that property taxes are an example of a tax on unrealized gains. Everyone whose home has gone up in market value knows this because their property tax has surely increased over the years even though they haven't sold their home.

Land Value Tax (the Georgeism discussed in the article) just taxes the land on what it is worth, not the buildings on it. So holding land vacant costs more because the speculator is not making any money from it, while they could be making money from a building.

A side benefit for homeowners is that improvements increase equity and resale value without raising taxes