XSLT 1.0 is still available on major browsers but some smartphones don’t have it. Some browser vendors would be happy removing their XSLT engine just to gain memory. They usually argue that XSLT 1.0 is not significantly used in websites. And Mozilla Firefox own XSLT 1.0 engine bugs won’t be fixed…
HTML, CSS and Javascript have evolved significantly to HTML5, CSS3 and ECMAScript 6 and browsers support many of their features which allows more powerful programming.
Machines are faster and Javascript is more and more optimized for performance.
Vanilla Javascript can still be a pertinent choice because even once famous Javascript frameworks come and go while others don’t bother with ascending compatibility… They do not seem to be really interested in performance either.
By the way, Node.js appeared so vanilla Javascript can also run outside a browser. Just using it, it is very easy to run an HTTP server on a local machine: Node.js can be used to interact with files, databases and so on while XForms is interacting with users.
All these evolutions allow XSLTForms modernization to be declined in the following axes:
An HTML5 notation for XForms elements and CSS styling of them
An XQuery/XPath 3.1 parser written in Javascript
An XQueryX 3.1 to Javascript transpiler
XForms 2.0 [XF2] is also in good progress and XSLTForms should support all its features as well.