JOOMLA ONLY
Monitors the website via cURL, checking its HTTP status code (see HTTP status codes for reference). A response in the 4xx-5xx range means the site is not reachable by the public; the site is then marked offline and notifications start going out.

Two checkboxes control it per site:
The offline history table lists every past outage, with the HTTP code returned, the error message, and the timestamp.
This feature requires a scheduled task, configured one of two ways:
php /path_to_your_joomla_installation/cli/joomla.php controlcenter:monitor.A run every 10 minutes is a reasonable starting point.