xsl:fallback
Used to define recovery action to be taken when an instruction element is used in the stylesheet and no implementation of that element is available.
Category: instruction
Content: sequence-constructor
Permitted parent elements: 
         xsl:analyze-string
         ; xsl:evaluate
         ; xsl:fork
         ; xsl:merge
         ; xsl:next-match
         ; xsl:try
         ;  any XSLT element whose content model is 
        sequence-constructor; any literal result element 
      
Element has no attributes
Saxon availability
Available in XSLT 1.0 and later versions. Available in all Saxon editions. Available for all platforms.
Details
The xsl:fallback element is used when a stylesheet contains an
                    instruction element and no implementation of that element is available. An
                    element is an instruction element if its namespace URI is the standard URI for
                    XSLT elements or if its namespace is identified in the
                        extension-element-prefixes attribute of a containing literal result element, or in the
                        extension-element-prefixes attribute of the xsl:stylesheet element.
If the xsl:fallback element appears in any other context, it is
                    ignored, together with all its child and descendant elements.
If the parent element can be instantiated and processed, the
                        xsl:fallback element and its descendants are ignored. If the
                    parent element is not recognised or if any failure occurs instantiating it, all
                    its xsl:fallback children are processed in turn. If there are no
                        xsl:fallback children, an error is reported.