WebMonotonicClock

performance.now(), which is the page's monotonic clock.

Monotonic by specification and unaffected by wall-clock changes, which is the property the engine's sync needs. Its resolution is deliberately coarsened by browsers against timing attacks (typically to 100 microseconds, more when the page is not cross-origin isolated), so this is a millisecond-scale clock wearing nanosecond units, and the engine's tolerances are far wider than that.