The double-colon is probably a necessity to disambiguate a scalar from an inline list that only has one scalar.

For example `x: 3` would be equivalent to `"x": 3` in JSON, but `x:: 3` is equivalent to `"x": [3]`

Maybe author of huml will read: I would prefer mandatory braces for inline stuff. “X:: 3” feels like a trick question.

A trailing comma would also solve this, but is equally awkward. A clear list delimiter would work for both machines and humans.