'resources/src/mediawiki.skinning/messageBoxes.css' classes are from early times in MediaWiki core, where CSS class architecture wasn't yet a thing.
The classes should be turned into
- messagebox => mw-message-box
- errorbox => mw-error-box
- warningbox => mw-warning-box
Without strong opinion if *-box is the right naming scheme, we're using *-container in a related, but slightly different context.
There would be also the possibility to use mw-error and mw-error--inline assuming that user notice boxes are the default way of exposing them.
- Update includes/Html.php to add the new classes as above
- Update resources/src/mediawiki.skinning/messageBoxes.less to use the new classes, but retain the existing classes with a comment explaining that these are supporting on-wiki content.
- Update Vector to use the new method.
- Write a Tech News entry (or write a draft and a desired announcement-time as a comment below)
- Update the non-standard uses of success, errorbox and warningbox class to use the new methods
- Drop CSS for successbox, errorbox and warningbox classes in core (April 2022)
Go to the login page on desktop and login with a bad username/password.
Expected: A box with a colored background should appear prompting you of your error.
Repeat for mobile (Minerva skin)