On my iPhone and MacBook, the practice routines on the website don’t stop the devices from going to sleep - which then stops the countdown timer, and the metronome (IIRC). The app handles this correctly, ie the screen won’t lock mid-song.
It’s a minor inconvenience, and I hesitated before posting here, but it looks like a fairly simple JavaScript fix, according to my friendly neighborhood CoPilot 
. Could the web dev team take a look?
Thanks for sharing!