To avoid losing CSS styling of message Please see https://doc.wikimedia.org/codex/latest/components/demos/message.html#css-only-version for the updated HTML markup you should be using.

This is a great way to ensure that styling is lost on any skin other than Vector 2022 and Minerva, immediately rather than within 30 days. Compare the original version with the “fixed”, i.e. broken one on Vector 2010. Could you please, please not break wikis without a suitable replacement being provided?

In T374499#10146580, @Tacsipacsi wrote:

To avoid losing CSS styling of message Please see https://doc.wikimedia.org/codex/latest/components/demos/message.html#css-only-version for the updated HTML markup you should be using.

This is a great way to ensure that styling is lost on any skin other than Vector 2022 and Minerva, immediately rather than within 30 days. Compare the original version with the “fixed”, i.e. broken one on Vector 2010. Could you please, please not break wikis without a suitable replacement being provided?

You shouldn't be using system message boxes for non-system messages as it is important these are distinguishable from messages in content - we don't want vandals using them for phishing attacks for example. These are not currently marked as a stable interface. As noted in T300314 a lot of usages relate to the historic confusion of messagebox's introduction (context: T300306#7699797) the correct fix here is to move away from that markup to a local template - in this case https://meta.wikimedia.org/wiki/Template:Message looks appropriate.

This was already "broken" in Minerva skin and for Vector 2022 with JS disabled, and there have been messages in the JS console about this for some time:
https://meta.wikimedia.org/w/index.php?title=Tech/Header&oldid=26342327&useskin=minerva

In T374499#10149891, @Jdlrobson wrote:

These are not currently marked as a stable interface.

Which doesn’t mean you have to break them. And definitely doesn’t mean you should propose a replacement that silently fails on some skins.

the correct fix here is to move away from that markup to a local template - in this case https://meta.wikimedia.org/wiki/Template:Message looks appropriate.

Local templates are a huge tech debt. They have to be kept up to date on each wiki one by one (wastes a lot of human time) yet are bound to be different across wikis and out of date. Your proposed template

• doesn’t have any interwikis, i.e. it’s so different across wikis that it doesn’t exist on any other wikis at all (or at least it’s not discoverable);
• doesn’t work as expected on narrow screens (the horizontal margins and the icon on the left remain no matter how narrow the screen is, leaving too little space for the text);
• doesn’t work as expected in night mode (remains dark-on-light).

Yes, it’s probably possible to fix these on Meta. And on mediawiki.org if a similar template is created there. And on Commons. And on English Wikipedia. And on French Wikipedia. One by one. I won’t, I have no time for that.

You shouldn't be using system message boxes for non-system messages as it is important these are distinguishable from messages in content - we don't want vandals using them for phishing attacks for example.

After deleting the styles, the only thing that will happen is that almost every project's Common.css will have its own (with varying degrees of obsolescence) copy of these styles. It will not appear in some, and the messages will look bad.

These styles have been used for many years, and it is strange to expect that communities will suddenly decide to come up with their own set of styles for notifications.

And the argument about vandals is quite strange. You can make the same design simply using inline styles. And this does not limit vandals in any way, so only honest editors suffer in this situation.

The phishing attacks are not made more possible by the existence of .mw-message-box class. That argument is just false. By that point you might as well prohibit using TemplateStyles or inline styles as well, since they can replicate even Codex-looking boxes without much work (and currently anyone, including IP editors, can create TS pages on most projects, phishing galore). This change only makes it harder for legitimate users to have warnings/errors/etc. without reimplementing the same markup for the 1000th time. To this end, it should be reversed and completely abandoned as a plan, as the previously planned deprecation of .error class was.

There is also an additional problem where Codex styles do not always get loaded reliably for every user group, see task and comment in T373602: FlaggedRevs Codex styles display unstyled in some conditions.