Google Code-in

This task is about MediaWiki itself.

Codepoint: Search for editingold in the file https://gerrit.wikimedia.org/g/mediawiki/core/+/master/includes/EditPage.php

Wrap in either warningbox or mw-warning CSS class. See Demo of both those/sandbox&oldid=2860082). Probably warningbox unless there's guidance somewhere (I can't find docs for that).

You are expected to provide a patch in Wikimedia Gerrit. See https://www.mediawiki.org/wiki/Gerrit/Tutorial for how to set up Git and Gerrit.