Thread:NoWayThisUsernameIsAlreadyOwnedBySomeone/@comment-24335865-20150613194744/@comment-26143371-20151002194114

Yup.

I've done my edits now (you probably have seen it). Now it still needs approval before it goes live to all users. You can enable "test mode" if you want. Go to either MediaWiki:Common.js or MediaWiki:Tabber.js and click "Enter Test Mode". After a few refreshes to clear the cache you should be able to see both your username template and the tabber in previews working.

PS USERNAME won't work in the main-namespace, I purposely disabled it there!