I believe the spec is at XSLT 3.0 but no browser actually implemented past XSLT 1.0 (not 100% sure - almost nobody cared about this feature last month so hard to find good docs on support). HTML5 and C++ are cut from the same cloth - massive and no reference implementation so full of features that have been “standard” for 10 years but never implemented by anyone.

Yeah, sorry, the XSLT spec is at 3.0 right now of course, but the browsers don’t implement it, and the WHATWG HTML Living Standard only mentions XSLT 1.0.

even outside of browsers barely anything supports XSLT newer than 1.0