I might be able to answer my own question here.
Edit: Looks like it doesn't play unless I tap on it to open. Did I do something wrong or is that just a feature of Lemmy?
It autoplays for me in multiple browser UIs (lemmy-ui, Photon, Tesseract, Alexandrite) as well as in the Summit, Voyager, and Thunder mobile apps. Though the frame rate is ridiculously fast in Thunder for some reason.