From Xojo Documentation
Returns True if the current instance implements the specified event.
This method is very important to the efficient operation of the Web Framework. It is designed to be a way to detect whether or not a developer using a control actually has the need for a certain kind of data from a control at runtime. For example, if you were creating a signature capture control using a canvas control, while there may be some worth to getting every "pen stroke" for some users, others might just want the resulting signature. By not implementing the MouseMove event, there is no reason to even send those events to the server.
Assuming you were creating a class named HTMLArea, you could find out if the user had implemented the MouseMove event like this: