Crossbrowser equivalent of explicitOriginalTarget event parameter

There is no equivalent to explicitOriginalTarget in any of the other than Gecko-based browsers. In Gecko this is an internal property and it is not supposed to be used by an application developer (maybe by XBL binding writers).

2015 update... you can use event.relatedTarget on Chrome. Such a basic thing, hopefully the other browsers will follow...

The rough equivalent for Mozilla's .explicitOriginalTarget in IE is document.activeElement. I say rough equivalent because it will sometimes return a slightly different level in the DOM node tree depending on your circumstance, but it's still a useful tool. Unfortunately I'm still looking for a Google Chrome equivalent.