The DocBook XSL Stylesheets include a set of Java extensions for XalanJ version 2. Extensions are not provided for XalanC at this time.
The extensions are included in the distribution in extensions/xalan2.jar. Xalan 1 is not supported.
Just make sure that the jar file is in your CLASSPATH when you run Xalan.
For compatibility with other processors, the extensions are disabled by default. To enable the extensions, turn on $xalan.extensions, for example by passing -PARAM xalan.extensions 1 to Xalan.
For more control over the specific extensions, see HTML Parameter Reference.