If they were going to implement it themselves it would be unnecessary to faff around with webassembly. They could just keep the native code implementation that they already have.
The problem is that XSLT has so few users that the browser writers don't want to waste any more time on it. Those few people who wrote their site around it will have to either rewrite it to use something else or add some third party webassembly implementation themselves.
The problem is that XSLT has so few users that the browser writers don't want to waste any more time on it. Those few people who wrote their site around it will have to either rewrite it to use something else or add some third party webassembly implementation themselves.