Bug 70672 (T72672) fixes the security hole of allowing Javascript in CSS in the Mediawiki namespace. It does this by breaking the functionality of loading CSS when on the Special:UserLogin and Special:Preferences pages. Unfortunately this means that any custom styles are not loaded. To the end user it causes momentarily confusion that they may have been maliciously redirected to a different site to enter their username and password. This is an undesirable side effect for the user interface.
I have created an example extension that will prevent saving any custom CSS that contains Javascript imports.
https://github.com/Alexia/Bug70672
Example error output:
http://imgur.com/a/TnsTY#0
Original bug:
https://bugzilla.wikimedia.org/show_bug.cgi?id=70672
Version: 1.23.5
Severity: major
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=70672