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The system identifier, a URI reference , for this input source. The
system identifier is optional if there is a byte stream or a
character stream, but it is still useful to provide one, since the
application can use it to resolve relative URIs and can include it in
error messages and warnings (the parser will attempt to fetch the
ressource identifier by the URI reference only if there is no byte
stream or character stream specified).
If the application knows the character encoding of the object
pointed to by the system identifier, it can register the encoding by
setting the encoding attribute.
If the system ID is a relative URI reference (see section 5 in ),
the behavior is implementation dependent.