onBeforeUpdate
The instruction(s) to be executed are placed within quotes. This event handler will execute the instruction(s) when data from an element is about to be sent to a data source.

The following example will display text of your choosing in a message box when data from an element is about to be sent to a data source:
onBeforeUpdate="msgWindow.document.write('-- Insert text message here --')"
This Event Handler is used with the following:

APPLET
AREA
BODY
BUTTON
CAPTION
DIV
FIELDSET
IMG
OBJECT
SELECT
TABLE