XSLT is kinda of horrible

The aesthetics aren't really part of the argument, XSLT is an open standard with multiple implementations which exists, and is built for a different vision of the web built out of information sharing than the dominant advertising and surveillance supported one built by big tech companies. Using XSLT to build a web view client-side means that the client has access to the rich XML data too, with the great example of RSS, which is not tied to any particular vendors software stack (get your podcasts *wherever* you get them), which is a very radical feature given how much of the modern web is driven by FAANG, which uses web technologies, like JSON and JavaScript, but tends to be locked in bespoke APIs and difficult to maintain frameworks which limit integration to large companies which can afford it. This isn't a technical argument about the merits of XSLT, it's mourning of a way of being. RETVRN ;-) There is some technical meat though, if the XSLT engine had been maintained, if Apple and Google or MS had thrown a few FTE to the project then maybe it could have been updated to XSLT v2 and v3 which supports templating JSON data as well, and that have might lead to a very different kind of design for PWA and SPA than what React (Facebook) and Angular (Google) evangelized, none of which works toward the strengths of the browser engine or the web as an information-sharing platform, SPAs tend to obfuscate the underlying information and do not lend themselves to custom user-agent behavior in the way RSS does.

This whole argument is difficult to articulate because it's an emotional, vision and alignment discussion and not primarily a technical one