Stopwatch

The first idea that comes to mind when you build a stopwatch is to have setInterval bump a counter every tenth of a second. But the setInterval delay is a minimum, not a guaranteed schedule: ticks run late when the page is busy, and browsers slow timers in background tabs down to one tick per second, so ten minutes in the background turn into one minute on the counter.

The right way is to save the start time with Date.now(), then work out the elapsed time by subtraction every time you display it. The timer's only job is to decide when the screen gets updated, not to measure how much time has passed.

Watch out: to resume counting after a stop, you need to keep the time that built up before it and add it to the new stretch. And prevent double starts, because clicking "Start" twice creates two timers, and only one of them ever gets cleared.