Netscape has never supported external IDs/URLs in the USEMAP attribute to the IMG element as per the specification (only internal anchors can be used.) Internet Explorer supports external IDs/URLs from version 1.0 to 3.0, now it does not support them. Opera has supported external IDs/URLs since version 3.0.

IMG reference, with a map definition in an external document (map-externalmapdef.htm#myimagemap):
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Shape destinations on image: