On Fri, 9 Aug 2019 22:57:05 +0200, Mike Wey wrote:

Gtk treats any value other NONE as the same value, so it's like a
boolean but with more than one value meaning true.

You're probably answering these other questions with this reply, but just to clarify...

Is this behaviour specific to GtkD or is it all implementations of GTK?

Will it these style values have the intended effect in 4.0? In other words, will they work?