Any number of
user-defined attributes may be defined on xsl:output. These
attributes must have names in a non-null namespace, which must not be either the XSLT
or the Saxon namespace.
The value of the attribute is inserted into the Properties object made available to
the Receiver handling the output; they will be ignored by the standard output methods,
but can supply arbitrary information to a user-defined output method. The name of the
property will be the expanded name of the attribute in JAXP format, for example
"{http://my-namespace/uri}local-name", and the value will be the value as given in the stylesheet.
