I wanted to know if group instances from a blend file are compatible with animation, so that my group instance models (animated thru armature) stored in a different scene will be played in my main scene when I send it sketchfab.
I found out that it works: group instance in my main scene remain animated in sketchfab. But you need at least to have an object in your main scene which ist not a group instance.
If you have only group instances in your main scene, it will not work.
You'll get an error message on sketchfab viewer: "The file is invalid (Error 13). Please check the file or use an exporter to ensure compatibility".
You can try for yourself by watching and downloading my test blend file.