Logging with XSLT

Question

How and where could I output log messages for debug and performance purposes during an XSLT transformation?

I guess the simplest method is using express

Answer 1

By looking for a solution to this problem, I ended up implementing a logging mechanism in XSLT in similar fashion to log4j, using mostly xsl:message and pure XSLT 2.x. I took some of the answers in this page as inputs. The library is available here: https://github.com/ukuko/log4xslt