Workspace rules: on-created-empty

[pescepalla]

The Workspace Rules page of the wiki seems to suggest that it is possible to apply rules to automatically created windows in special workspaces if putting them in square brackets: workspace = 5, on-created-empty:[float] firefox

However I cannot seem to make this work. For example I have a workspace rule I use to create a "dropdown" terminal in conjuction with a keybind: workspace = special:, on-created-empty:[opacity 20%] footclient

Here, the workspace with the terminal window is correctly created, but it is not semitransparent.

Other window rules are also ignored, including float. I wonder what I am doing wrong here.