Thanks to Ephemeral
here is the answer:
to disable the X dependency issue, I added environment="JACK_NO_AUDIO_RESERVATION=1" in the systemd config
pi@raspberrypi:~ $ cat /etc/systemd/system/jackd.service
Description=Jack audio server
I have just checked your test video and it works fine in the latest Pale Moon, with sound.
Pale Moon browser is a fork of Firefox which supports older Firefox addons (which Firefox dropped after version 52). It is slower than Chromium but requires significantly less RAM, so it can actually outperform Chromium on a system which is short on memory.
Pale Moon ...