Checking comment made in WebmasterWorld JS Forum:
About the only thing I can offer is that with onmouseover, onclick should be redundant, since one would have to mouseover to click. Still, the onclick event is more reliable than onmouseover, which can fail at times.
Onmouseover and onclick are both mouse events; onfocus would cover the case where a field was tabbed to and the enter key used simulate onclick.Rambo Tribble
Move to the link below using the keyboard and try activating the link using the keyboard only - does the onclick event fire?
The onclick event is triggered! It is not just a mouse event in this respect - or maybe the onclick event is simulated for accessibility?
NB: The onfocus event in the above DIV does fire in IE6 (not FF) when the user clicks on it (immediately before the onclick event). The user cannot, however, cause the onfocus event to fire using the keyboard (tab).