<input type="checkbox" onclick="handler"> checkbox.onclick
The onclick property of a Checkbox object refers to an event handler function that is invoked when the user clicks on the checkbox. See HTMLElement.onclick for complete details. Note, however, that Checkbox.onclick has been supported since JavaScript 1.0, unlike the generalized HTMLElement.onclick handler.
HTMLElement.onclick; Chapter 19; EventListener, EventTarget, and MouseEvent in the DOM reference section