The onfocus property returns the onFocus event handler code on the current element.
element.onfocus = event handling code
The focus event is raised when the user sets focus on the given element.
Opposite to MSIE, which almost all kinds of elements receive focus event, almost all kinds of elements on Gecko browsers do NOT work with this event.
|WHATWG HTML Living Standard
The definition of 'onfocus' in that specification.