Namespace declaration pseudo-attribute for XLink.
This attribute value is fixed and will be added implicitly by many processors. For data portability, however, it is recommended that it be given explicitly, in order to enable a document to be processed standalone.
Strictly speaking this pseudo-attribute (namespace declaration) is not needed by documents that contain no links, but these will be rare.
|http://www.w3.org/1999/xlink||The prefix “xlink” has been set to this URL, the URL of the W3C XLink Recommendation.|