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Non-standard
This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
Summary
The original target of the event before any retargetings. (Mozilla-specific)
In presence of XBL anonymous content this will be the anonymous node the event originally fired on. See Anonymous Content#Event_Flow_and_Targeting for more details.
Note: originalTarget may also be native anonymous content (see bug 208427), in which case it's useless for non-privileged code.
See also Comparison of Event Targets
Example
Need an example that makes sense here
Specification
This is a Mozilla-specific property. Defined in /dom/public/idl/events/nsIDOMNSEvent.idl
This event property is not defined in the W3.org DOM Level 2 Events