Navigator 2.0, Internet Explorer 3.0
The alert() method displays the specified message to the user in a dialog box. The dialog box contains an OK button that the user can click to dismiss the dialog box.
Perhaps the most common use of the alert() method is to display error messages when the user's input to some form element is invalid in some way. The alert dialog can inform the user of the problem and explain what needs to be corrected to avoid the problem in the future. The appearance of the alert() dialog box is platform-dependent, but generally it contains graphics that indicate that the message indicates an error, a warning, or an alert of some kind. While alert() can display any desired message, the "alert" graphics of the dialog mean that this method is not appropriate for simple informational messages like "Welcome to my home page" or "You are the 177th visitor this week!"
Note that the message displayed in the dialog is a string of plain text, not formatted HTML. You can use the newline character, \n, in your strings to break your message across multiple lines. You can also do some very rudimentary formatting using spaces and can approximate horizontal rules with underscore characters, but the results you achieve will depend greatly on the font used in the dialog, and will thus be system dependent.