Well I can’t play any of the media, perhaps JavaScript isn’t that bad?
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.