As others have pointed out there are multiple implementations of XSLT, but I’d also argue that this situation seems like a decent argument in favor of that policy. If everyone is using a single implementation then in practice that implementation is the standard and things like Hyrum’s law become serious considerations.

XSLT is grandfathered in from the early days of the web, and while it’s turned out better than Microsoft exposing random COM interfaces which even Windows developers hated it’s still something of a cautionary example of a feature which never really caught on but browser developers have to support decades later or be willing to break a modest number of sites, some relatively important in particular niches like government information. I think of what happened with WebSQL as a reaction to the maintenance costs of a decade earlier.