The <xsl:fallback> element specifies what template to use if a given extension (or, eventually, newer version) element is not supported.
<xsl:fallback> TEMPLATE </xsl:fallback>
None.
None.
Instruction, appears within a template.
XSLT, section 15
Not supported at this time.
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https://developer.mozilla.org/en-US/docs/Web/XSLT/fallback