I would rather that they introduced support for v3, as that would make it easier to serving static webpages with native support for templating.
What exactly is the difference between generating HTML using the browser's XLST 1.0 runtime and SaxonJS's XLST 3.0 runtime? Before you say the goal is to not have to deal with JS, then you've already accomplished that goal. You don't need to touch NPM, webpack, React, JSX, etc.
Blocking first party JS is lunacy by the way.
Plug: here is a small project to get the basic information about the XSLT processor and available extensions. To use with a browser find the 'out/detect.xslt' file there and drag it into the browser. Works with Chrome and Firefox; didn't work with Safari, but I only have an old Windows version of it.
I updated an XSLT system to work with then latest Firefox a couple of years ago. We have scripts in a different directory to the documents being transformed which requires a security setting to be changed in Firefox to make it work, I don't know if an equivalent thing is needed for Chrome.
It wasn't that bad. We used tomcat and some apache libraries for this. Worked fine.
Our CMS was spitting out XML files with embedded HTML that were very cachable. We handled personalization and rendering to HTML (and js) server side with a caching proxy. The XSL transformation ran after the cache and was fast enough to keep up with a lot of traffic. Basically the point of the XML here was to put all the ready HTML in blobs and all the stuff that needed personalization as XML tags. So the final transform was pretty fast. The XSL transformer was heavily optimized and the trick was to stream its output straight to the response output stream and not do in memory buffering of the full content. That's still a good trick BTW. that most frameworks do wrong out of the box because in memory buffering is easier for the user. It can make a big difference for large responses.
These days, you can run whatever you want in a browser via wasm of course. But back then javascript was a mess and designers delivered photoshop files, at best. Which you then had to cut up into frames and tables and what not. I remember Google Maps and Gmail had just come out and we were doing a pretty javascript heavy UI for our CMS and having to support both Netscape and Internet Explorer, which both had very different ideas about how to do stuff.
??
I was transforming XML with, like, three lines of VBScript in classic ASP.
You needed the jvm and saxon and that was about it...