This attribute may be used to create three state buttons, numbered 0, 1 and 2. When in state 0 or 1, pressing the button will switch to the opposite state. When in state 2, pressing the button will switch to state 0. This means that the button acts like a checkbox except that there is a third state which must be set manually by adjusting the check state. If you wish to have different behavior for how the states are adjusted, set the autoCheck attribute to false and adjust the state with a script. The type attribute must be set to checkbox for buttons with a check state. Constants for the possible values for this attribute are in the nsIDOMXULButtonElement interface.