Not working for me on Safari Version 14.0 (15610.1.28.1.9, 15610) Catalina 10.15.7 (19H2)
Working on Chrome Version 86.0.4240.80 (Official Build) (x86_64) same machine
Working on Chrome Version 86.0.4240.80 (Official Build) (x86_64) same machine
On Chrome, the mute button defaults to the muted state and never responds to a click. Pause/play always works to toggle the animated display with every click.
On Safari, the pause button is unresponsive. The mute button toggles the animated display with every click, and changes state between muted and unmuted. When muted, animation is paused. When unmuted, animation is active. When muted and animation paused, a click on the play button does not change the paused animation state, but does result in the triangle symbol displaying smaller during button down, then restore to larger when button up. Sometimes I can get the play button to work (toggle animation on/off) after clicking in the progress bar a number of times and toggling the animation state that way, after which (sometimes) the play button works, but not always.
Also, the display of controls is different on Chrome than Safari.
Chrome, progress bar is a full width bar underneath the other controls.
Safari, progress bar is in line with the other controls, which include -15 and +15 either side of Play. Those skip controls are not present on Chrome.
It sounds like you're describing some cross browser issues playing that mp4 file.
Thanks for the feedback though. I'll try to reword that section so it's more clear.