> which modern browser doesn't handle XSLT 1.0?
I know it depends of the definition of "modern browser", but I'm fairly sure that MSIE and pre-chrome Edge had lost support for it at some point, since MSXML stopped being shipped with them.
I honestly don't know what the current state of the art is, but Firefox doesn't do native XSLT anymore afaik (yes, you can do it in JS, but it's an extra step).
Nor did the spec itself stop evolving. The problem is that the browsers have deprecated the technology before XSLT 2+ could be adopted.