[asknthrow]

I don't think this is correct. AFAIK they both run in the UI thread (which is why running your timer in a webworker turns out to be more accurate as only the callback is affected by queue length of scheduled calls in the UI thread). Although this article https://www.html5rocks.com/en/tutorials/audio/scheduling is about sheduling audio specifically, it is the go-to resource for accurate web timing (and although not explicitly mentioned, the metronome is supposed to run in a webworker).