Comment by Mikhail_Edoshin

In Saxon you write templates that give you partial results and just call them with the -it:name switch. So if you have a four-step transform you can examine the results of each step in isolation. In libxslt that only supports 1.0 you can do this with parameters, although this is less convenient.

You can trace with xsl:message. Step-by-step debug is not there, granted; but it is definitely the last resort tool even when it is available.