When the "Alertify notifications" swtich is enabled at Admin -> Settings -> Switches (available since WSN 8.0.0), incomplete submission notices and redirect pages after submissions will be replaced with javascript-based alerts in the style displayed at http://fabien-d.github.io/alertify.js/ using that javascript library. The result is fancier and perhaps more user friendly, but also more demanding on the viewer's computer.

Please note that when a regular html file upload field is present on the page (as it is on the edit forum page for the special forum icon field, and as you may have for any custom file fields you create) the form will submit the old fashioned way instead of using alertify. This is because alertify's AJAX submission method would not be able to upload a file. Pages that use the bulk uploader (submit/edit topic, send private message, post post) don't have this problem since the bulk uploader uploads the files before the form is submitted.