Navigator 2.0, Internet Explorer 3.0
The onunload() event handler provides the opportunity to perform any necessary "clean-up" of the browser state before a new document is loaded. For example, an onunload() handler might restore the Window.defaultStatus property (the default message in the status line) to the empty string.
When the browser leaves a site using frames, the onunload() handlers for each frame will be invoked before the onunload() handler of the browser itself.
The onunload() handler is invoked when the user has instructed the browser to leave the current page and move somewhere else. Therefore, it is almost never appropriate to delay the loading of the desired new page by popping up dialog boxes (with Window.confirm() or Window.prompt() for example) from an onunload() event handler.