> What exactly is the difference between generating HTML using the browser's XLST 1.0 runtime and SaxonJS's XLST 3.0 runtime?

Several hundred kB (compressed) of runtime, for one. It could make sense for browsers to have something like that built-in like they did with pdf.js, though Saxon is proprietary so it would not be that thing.

I speak only for my self, but I greatly value having no dependencies or build processes. Just put the files on a web server and have it run for the next 20 years.

It might not scale for larger businesses, but for regular people on the web who just want to put something out in the world and have minimal churn keeping it up, it can have great value!