Browser notification
A browser notification is a message from a website or web app displayed through the device's notification system, subject to the browser's permission settings and platform support.
A website’s message can look like a system alert
Web notifications use the operating system’s notification interface. Their appearance outside a browser tab does not make their contents an operating-system diagnosis. A site can supply a fake warning using the same channel a legitimate web app uses for a message alert.
Microsoft documented sites that asked people to allow notifications under the pretext of proving they were human, then sent deceptive security warnings. The permission was real; the explanation for requesting it was misleading.
Permission is not the notification itself
Allowing notifications authorizes a channel. It does not establish that any particular notification arrived or that someone acted on it. Persistent notifications can remain interactive after an individual page closes, while other notifications are tied to the page’s lifetime. Behavior depends on the browser and implementation.
For an unwanted warning, inspect the originating site and use the browser’s notification settings to block that site’s messages. Avoid following a repair or purchase link inside the warning. The notification-permission article explains the support handoff.
Blocking the channel does not undo a payment, download, or command already performed. Report those separately. If the message persuaded someone to execute instructions, the ClickFix article covers that additional step.