The web is what the engines implement and what sites use. Always has been. A standard that's not implemented isn't worth anything.
Web standards exist to help browsers be interoperable. Before them one browser would implement something, and if another liked it they would too. That can still happen, but standards mediate the process to be less chaotic and more reliable.
Web standards cannot force browsers to do anything. If a browser doesn't implement a standard - and there are lot of unimplemented "standards" out there - then it just doesn't.
As a user then you have the choice to use browsers that offer better or different standards support. If you require XSLT support, and Firefox removes it, you can use Safari, or Chrome. If Chrome removes it, you can hopefully use an Blink engine that keeps the feature on (usually they're removed with a flag far before they're actually deleted with code).
And this is the real problem with a lack of browser diversity: Users need to be able to vote with their feet.
But... if all the vendors agree on something, as a user good luck with finding an alternative. I seriously doubt Opera, Brave, or Vivaldi is going to do the work and take the security risk to add back XSLT. Servo and Ladybird aren't going to come to the rescue here either. They most likely would love to have less to implement.
