Standard: http://www.w3.org/TR/xslt#fallback

fallback

Attributes:

Standard excerpt:

Normally, instantiating an xsl:fallback element does nothing. However, when an XSLT processor performs fallback for an instruction element, if the instruction element has one or more xsl:fallback children, then the content of each of the xsl:fallback children must be instantiated in sequence; otherwise, an error must be signaled. The content of an xsl:fallback element is a template.