I too spent a chunk of time seeing what worked and what it looks like…

JS referenced by the XML can manipulate the XML but it frequently executes before the XML DOM is ready (even when waiting for onload) and so misses elements

So while possible it’s a pretty horrible experience to translate XML to HTML using JS - the declarative approach is more reliable and easier IMV

The XSLT polyfill doesn’t seem to work when loaded as a script in an XML doc but not quite sure why ATM

application/xml is commonly used for RSS feeds on static hosts because it’s the correct mimetype for say a feeds.xml response