Firstly make sure you have  xmlns="http://www.w3.org/1999/xhtml" on your
xsl:stylesheet so it is in scope in the whole stylesheet.

then,
If you are generating the elements, just use the unprefixed form in the
stylesheet, if you are copying then from elsewhere and they are alreay
prefixed then instead of
<xsl:copy>
use
<xsl:element name="{local-name()}">

David