Comment by xg15

True, but that raises the question, why don't the browsers do that? I think no one would object if they removed XSLT from the browser's core and instead loaded up some WASM/JavaScript implementation when some XSLT is actually encountered. Sort of like a "built-in extension".

Then browser devs could treat it like an extension (plus some small shims in the core) while the public API wouldn't have to change.