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

A simple hack would be to create a variable with xsl:variable and to either just concat() new values to it or to set up a xsl:template which does the same thing. Then you just need to output this variable at the end of the execution and you can explicitly choose where to show the log.