Sounds like a Safari/Apple issue, not a lack-of-javascript issue.
Which video codec? Originally webm could contain only VP8, but nowadays it can also contain VP9 and AV1. It's possible that Apple only implemented VP8 (https://caniuse.com/webm implies that it has implemented VP8 and VP9 but has VP9 disabled by default); if this video uses a newer codec, it won't work.