@Ric-Atkinson WebM is open source and free, MP4 requires paying a licence fee. WebM is a Google product which explains why it runs on chrome.

I expect that many of the animations (many trials) are not recorded, hence it makes sense to work with an entirely free format and only move to MP4 requiring paying fees at user request.

Maybe you could try downloading in WebM to your own computer, and then converting to MP4 on your computer; There are a number of such convertors available on the ent.