You can create elements in a namespace either using a literal result element:
<h:html xmlns:h="http://www.w3.org/1999/xhtml"/>
(Here the namespace declaration of xmlns:h can be on the element itself or on any containing element in the stylesheet)
or using xsl:element
<xsl:element name="h:html" namespace="http://www.w3.org/1999/xhtml">
(If the namespace attribute is omitted, then the prefix "h" in this example is used to locate a namespace declaration in the stylesheet.)
You will find more information in any XSLT reference book (including, of course, my own!)
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference