Maybe prefer-reduced-motion?

Yep, that was it. This is not the first time a website failed to perform as advertised because of this. To be clear, I'm blaming the websites or their creators; this is not a bug, but rather a feature working exactly as intended. But I would recommend that if you're showcasing a feature that depends on a certain setting, and that setting isn't set, then you should show a warning of some sort (implementation details left as an exercise for the developer).