What does your browser do when you're not looking?

The Javascript code of this page schedules a function to be run every 10 seconds. The function requests a file from the server, registers that it was run and updates some statistics, for example how late the call was compared to the requested time.

When another browser tab or another app (on a mobile device) is opened instead of this tab, the browser runs (should run) a visibilitychange callback. This is recorded and logged.

While the tab is closed, it depends on the browser, whether and when the scheduled task is actually run. The task may run late, very late or even only when the browser tab is brought to the foreground again. The logging and statistics keeps track of what happens when.

Experiment a bit: open another tab or another app, wait a while and come back to see what happened and what not. If no scheduled calls are missed, it means that the page's activity is not reduced in the background. This should not happen in modern browsers.