Looks like there is a WASM polyfill for XSLT based on libxslt:

https://github.com/mfreed7/xslt_polyfill

To me this is the best way forward, I used to like XSLT quite a bit but the world has moved on and I get browser maintainers not wanting to try and maintain that extra native code surface area.

It seems like the browser should be incorporating many libraries as WASM to mitigate exploits and surface area such as XSLT and rarely used image formats etc. Then only do the native versions for widely used features where it's worth the effort to maintain and vet them.