For the next GTK cycle, Matthias has been working on a format for symbolic icons that can be rendered efficiently by GTK, including animations; it is based on a subset of SVG with custom vendor attributes:
discourse.gnome.org/t/animated…
If you want to experiment with content creation apps, or towards a shared implementation, feel free to drop by on Discourse.
Animated icons for GTK
I have been working for a while on GTK support for stateful, animated, symbolic icons. This post is meant to provide an introduction to the format, and encourage experimentation with it. The code will land very soon in GTK 4.21.GNOME Discourse