application/xml is not the same as application/rss+xml. application/xml also loads javascript just fine. again, i tested that. so far i have not found a single mimetype that can load xslt, but could not load javascript. i am coming to believe that there isn't one. if xslt works, then javascript works too.
whether javascript itself is a suitable replacement for xslt is not the question. your argument was that it is not possible to replace the builtin xslt support with anything written in javascript, because xml files can't load javascript.
since i have now verified that an xml file that can load xslt in the browser can also load javascript, this is proven wrong. all we need now is a good xslt implementation written in javascript or maybe a good binding to a wasm one and then we are ready to remove the builtin xslt support in the browser.