Extension elements in Saxon can be implemented only in Java. You must define a namespace that binds the extension to its implementation. However, the rules are more explicit here than with extension functions. The namespace must end in a /, followed by the fully qualified class name of a Java class that implements the interface com.icl.saxon.ExtensionElementFactory:

<xsl:stylesheet 
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:acmeX="http://www.acmeX.com/com.acemX.SuperExtensionFactory"
extension-element-prefixes="acmeX">
   
<!-- ... -->
</xsl:stylesheet>

The prefix must also be listed in the stylesheet’s extension-element-prefixes attribute.

Details of the ExtensionElementFactory are covered in Recipe 12.15.