Standard: http://www.w3.org/TR/xslt#element-attribute
attribute
Attributes: name, namespace
Standard excerpt:
The xsl:attribute element can be used to add attributes to result elements whether created by literal result elements in the
stylesheet or by instructions such as xsl:element. The expanded-name of the attribute to be created is specified by a required
name attribute and an optional namespace attribute. Instantiating an xsl:attribute element adds an attribute node to the containing
result element node. The content of the xsl:attribute element is a template for the value of the created attribute.