These fields may not be changed:

NS4/Unix: if you try long enough you may be able to slip in a few keystrokes
	  before the execution of onFocus..  but pretty unlikely
NS3:	you can edit things freely, onFocus has no effect
IE5:	works as it should

And here the status line should show when you use the selector box:
NS4/Unix: Starting to select seems to trigger both a Focus and a Blur,
	  the event handling here is pretty flaky..
NS3: no effect
IE5: works

According to HTML/4.0 you should see an alert when you hover over this link:

hurting link

NS4/Unix: no luck
NS3: no effect
IE5: neither

Untested HTML/4 features: area onFocus, label onFocus, button onFocus

See also body onFocus and frameset onFocus