Various XSLT processors provide various levels of debugging support. Many do not provide support for debugging of stylesheets, and thus can't be used for debugging. XSL Tools provides basic support for Xalan, and can be used as a guide for adopters on how to implement their own specific debugging support for other processors.
Xalan provides general stylesheet execution and evalation functionality. However there are a few things that Xalan does not suppor that other processors do.
Xalan Debugging Issues
Breakpoints - Xalan does not support stopping at Global Variables or Parameters when a debugging session is started. It will automatically start at the first template. Break points must be placed in templates to be honored. The reason for this is that Xalan lazily initializes the variables only when they are first used.
XSLT Text elements - Xalan does not send notifications of xsl:text elements. These are skipped over when debugging.
Built In Templates - Xalan does provide notification when the built in templates are called, but currently the debugger does not support or display stepping into these templates.