@dta_cthomas @emchateau In this case I was mostly digging up things that I already knew from about 15-20 years ago so I mainly used reference sites rather than tutorials. Mozilla Developer Network seems pretty good for that https://developer.mozilla.org/en-US/docs/Web/XML/XSLT
That said, I had a quick scan of M H Beals' "Transforming Data for Reuse and Re-publication with XML and XSL" on Programming Historian and that seems pretty good. It's focused specifically on #TEI too https://programminghistorian.org/en/lessons/transforming-xml-with-xsl
