Just like the current service=geoshape, users can now use service=geomask - works exactly the same way as geoshape, except that instead of highlighting specific regions, it allows users to highlight everything EXCEPT the specific regions.

For example, this masks out everything except Alaska (the example copied from the help page)

<mapframe text="Alaska" width=300 height=300 zoom=3 longitude=-152.58 latitude=64.01>
{
  "type": "ExternalData",
  "service": "geomask",
  "ids": "Q797"
}
</mapframe>

Screenshot: