When a message is displayed upon page load the alert will appear anchored in the upper left corner of the viewport or if on a mobile device the message will appear across the top of the viewport. (It will stay in place as the user scrolls the page.) Text in the upper left corner of desktop devices (rather than center) will help ensure screen magnification users do not miss the message upon page load.
This type of alert can always be dismissed by clicking on the close icon.
An example use case for this type of alert would be when the user clicks a confirmation link in their email which loads a new window confirming that their address has been verified.