Thanks, @GregRundlett, for resurrecting this very useful extension. I took a simple example and with #tag html it worked out of the box. I'm also happy it works with XML and XSLT stored on wiki pages if you add action=raw to their URLs.
Topic on Extension talk:XSL
Glad you found it useful.
If you feel like adding to the 'Usage' documentation to give an example of action=raw that would be great!
Sure! I added a brief example but feel free to modify.
I was wondering about "parse - if enabled the output of the transformation will be parsed as wikitext". Was the intended meaning here that wiki formatting, templates and parser functions would all be parsed? If so, that would be incredible (I'm thinking of possibilities it would offer for working with Semantic MediaWiki), but I'm afraid it didn't work for me.