The [[mw:Extension:GlobalCssJs]] extension allows for a user to specify custom JavaScript/CSS which will be loaded on all wikis where they have an account.
Most of the code is still in gerrit at I9329573da7d4f2af60515ef32b3b64bb769e3755.
Needs architecture and security review.
Version: wmf-deployment
Severity: major
URL: https://www.mediawiki.org/wiki/Extension:GlobalCssJs
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=29272
https://bugzilla.wikimedia.org/show_bug.cgi?id=14950
https://bugzilla.wikimedia.org/show_bug.cgi?id=68933