A form data set is a sequence of control-name/current-value pairs constructed from successful controls and if a form contains more than one submit button, only the activated submit button is successful while image creates a graphical submit button.

It does say: The submitted data includes name.x=x-value and name.y=y-value where "name" is the value of the name attribute, and x-value and y-value are the x and y coordinate values, respectively.

... but I can't see anywhere that it says the normal behaviour of other form controls (submitting the name/value pair) doesn't apply as well.