ah, it finally let me respond to this comment, sometimes hn is sooo weird :)
safari 14.1 in Mojave. I spend too much time with weird audio stuff to move past Mojave on this laptop unfortunately.
edit:
aha -> looks like you have to turn on the experimental features to get it to work. might be good to have a note for someone who doesn't have that turned on to avoid the black screen.
Been looking into it. Looks like it's an old safari issue related to Media Recorder (for downloading your sequence.) I'll play around to make sure the ux is right regarding this bug.
(editing to add, since I can't reply to the child for some reason):
Mojave, safari 14.1