I press play, then an spinning animation starts and it gets stuck in it.
I can reproduce this on Epiphany 3.36.4 (WebKitGTK 2.30.6), but can't reproduce it in MiniBrowser on latest WebKit (@276497, from Apr 23 2021). From the GStreamer logs, I can see that the webpage asks is the browser supports "video/mp4; codecs="hvc1"" and "video/mp4; codecs="av01.0.31M.08.0.110.01.01.01.0"", and both are unsupported in my local Epiphany. The webpage doesn't make any other attempt to ask for other codecs. However, in MiniBrowser (from Flatpak) "video/mp4; codecs="av01.0.31M.08.0.110.01.01.01.0"" *is* supported. The webpage also asks for "audio/mp4; codecs="mp4a.40.2"" and "video/mp4; codecs="avc1.4D401E"", which also are supported. The codecs used in the end by MSE are mp4a and AVC. I'm closing this bug as WORKSFORME.