There is a better alternative to libxslt - xee[1][2]. It was discussed[3] on HN before.
[1] https://blog.startifact.com/posts/xee/
[1] https://blog.startifact.com/posts/xee/
It's not just a matter of replacing the libxslt; libxslt integrates quite closely with libxml2. There's a fair amount of glue to bolt libxml2/libxslt on to Blink (and WebKit); I can't speak for Gecko.
Even when there's no work on new XML/XSLT features, there's a passive cost to just having that glue code around since it adds quirks and special cases that otherwise wouldn't exist.
My understanding is that browsers specifically use the 1999 version and changing this would break compat