11.6. <xsl:fallback>:
What to do if an extension is missing
If you are trying to use an XSLT extensions (see
extension elements,
below), and the extension can't be found, you can provide
an <xsl:fallback>
element to specify what actions should be taken. For
example, this fragment shows a reference to an
extension element named <exsl:document>. If that
element is not available, it writes a message and
terminates:
<exsl:document href="page.html">
<xsl:fallback>
<xsl:message terminate="yes">
Aieeee! The exsl:document package is missing!
I'm out of here!
</xsl:message>
</xsl:fallback>
</exsl:document>