Specifically, everything in http://mxr.mozilla.org/mozilla-central/source/dom/interfaces/events/nsIDOMNSEvent.idl except isTrusted really ought not to exist at all.
See also: http://lists.w3.org/Archives/Public/www-dom/2011JulSep/0039.html
https://bugs.webkit.org/show_bug.cgi?id=65283 is the WebKit bug.
originalTarget and explicitOriginalTarget are necessary for XBL 1 - they expose shadow content event targets.
We can maybe remove getPreventDefault once we implement defaultPrevented, unless sites are sniffing and relying on it, in which case it probably needs to be standardized as an alize for defaultPrevented.
The prevent* stuff has been deprecated for a while and I suspect can go.
The original target stuff is indeed needed for XBL1, but I wonder whether we can expose it to system-principal code only like we do baseURIObject and nodePrincipal. That ought to be good enough for our uses.
The various constants might need standandizing. Not sure.
(In reply to comment #4)
> We can maybe remove getPreventDefault once we implement defaultPrevented,
We already do, bug 656954.