If you're on linux, you most likely need the H.264 ffmpeg codec package and things like the Cisco OpenH264 plugin for Firefox. Also, try unblocking WebRTC if you block it
Still, only a black rectangle is shown (also disabled ad blocker).
There are a couple of _warning_ messages in the dev console though, which might explain the origin of the issue:
> An AudioContext was prevented from starting automatically. It must be created or resumed after a user gesture on the page.
> Autoplay is only allowed when approved by the user, the site is activated by the user, or media is muted.