[richard_todd]

I think the problem with XSLT is that it's only a clear win to represent the transform in XML to the extent that it is declarative. But, as transformations get more complex, you are going to need functions/variables/types/loops, etc. These got better support in XSLT 2 and 3, but it's telling that many xslt processors stuck with 1.0 (including libxslt and the Microsoft processors). I think most people realized that once they need a complex, procedural transformation, they'd prefer using a traditional language and a good XML library to do it.

I don't like seeing any backward compatibility loss on the web though, so I do wish browsers would reconsider and use a js-based compatibility shim (as other comments have mentioned).