On the fiddle, the first event has currentTarget equal to the element and the second has currentTarget equal to the form.

So the event is bubbling to the form, but shouldn't this be handled in the core? Only one element can have focus at the same time, so it doesn't make much sense to fire one for the input and one for the form.

Actually, the event is fired for each DOM up in the tree..

I don't see any reason why you can't just do: $(document.body).on('focus', 'input') or even better $('form').on('focus', 'input').

Mainly because elements other then form inputs that can have the focus and blur events (e.g. elements with tabIndex defined)

You could do: $('form').on('focus', 'input, [tabIndex]')

This is working correctly as far as I can tell. Delegated events only work with bubbling event types. The focus and blur events are not defined to bubble by the W3C, so **as a convenience** we translate them to focusin and focusout. There is no other special handling done for focus and blur.

As the event bubbles up from the input, jQuery compares each element to the selector and fires the handler on the element if there is a match. Since * matches everything, each element up the line gets its turn with the handler.

The effect is easier to see with a deeper tree: http://jsfiddle.net/dmethvin/fNEhG/4/

Or to avoid the red herring about the focus event, here's one with click: http://jsfiddle.net/dmethvin/fNEhG/5/