Standard: http://www.w3.org/TR/xslt#element-attribute-set

attribute-set

Attributes: name, use-attribute-sets

Standard excerpt:

The xsl:attribute-set element defines a named set of attributes. The name attribute specifies the name of the attribute set. The value of the name attribute is a QName. The content of the xsl:attribute-set element consists of zero or more xsl:attribute elements that specify the attributes in the set.