www.gtk.org GTK3 docs do not document style or child properties
The GTK3 docs pointed to by https://www.gtk.org/docs/apis/ (click "online" and then "GTK 3 API reference") are generated by GI-DocGen.
GI-DocGen does not intend to support style or child properties, and as such cannot document the whole GTK3 API.
As such, https://gtk.org should instead link the old, complete docs for GTK3. Linking incomplete docs as if they're authoritative is confusing and will lead to frustrated developers when they cannot find the API they need, or documentation for API surface that they see in use. GTK3 is still being used by a huge number of applications with ongoing maintenance and development, so it is important that the online documentation be complete.