The <xsl:fallback> element specifies what template to use if a given extension (or, eventually, newer version) element is not supported.
Syntax
<xsl:fallback> TEMPLATE </xsl:fallback>
Required Attributes
None.
Optional Attributes
None.
Type
Instruction, appears within a template.
Defined
XSLT, section 15
Gecko support
Not supported at this time.