Cancels the default action of an event.
Returns S_OK if successful, or an error value otherwise.
If an event is cancelable, the preventDefault method signfies that the event should be canceled. Any event handler can cancel the default action. If the event cannot be canceled, this method has no effect.
A call to this method is similar to setting the IHTMLEventObj::returnValue event property to VARIANT_FALSE in Internet Explorer 8 and earlier versions.