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The XSLT namespace has the URI http://www.w3.org/1999/XSL/Transform. It is used to identify elements, attributes, and other names that have a special meaning defined in this specification.
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
.............
Normally, the prefix xsl is used for referring to elements in the XSLT namespace. However, any namespace declaration that binds the prefix to the URI of the XSLT namespace is also valid.