Note:
This attribute sets a Uniform Resource Name (URN) for a target document.
URN is one way to locate a document on the internet.
The other way being by its URL.
To locate a document using its URN, the URN must be registered.
Approved namespaces are listed in the IANA registry.
see http://www.iana.org/assignments/urn-namespaces.
Syntax:
<a urn="value"> . . . </a>
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