So we are almost back to using XSLT (and this is a good thing)

Sorry, I'm struggling to see the comparison

No, and it wouldn’t be, respectively.

(Also, I don’t think we can go back to XSLT in the same sense that I can’t go back to the moon.)

Does XSLT support making HTTP requests to load new content into an existing page?

My first reaction, having done a fair bit of XSLT in ancient times, was "I hope not".

After a bit of searching all the examples I could see use JavaScript to glue the HTTP request making part and then invoke the XSLT processor so it looks like the answer is "no".