Can you provide some example urls / screenshots? Thanks in advance!

Try my attempt here: https://en.m.wikipedia.org/w/index.php?title=Wikipedia:Sandbox&oldid=884468638 (at the bottom of the page)

But it is broken in every hatnote on cswiki, enwiki and possibly all the others.

I think an assumption has been made here that hatnotes never contain links and if they do they link somewhere.

Flagging to Alex. I assume the issue here is that red links serve as an indication to editors that something broke.

If this was not intentional, the rule .hatnote a, .dablink a, .rellink a can be removed since that rule sets the hatnote link color to the default link color and is redundant. If it is intentional a comment should be added to the code.

@Jdlrobson @alexhollender_WMF Both assumptions are wrong.

1. Hatnotes do contain links in most cases. For example, [[w:en:Template:Main]] is used in >350,000 pages.
2. Links in hatnotes are frequently red links in some wikis, e.g. jawiki. This is because it is accustomed to include [[w:ja:Template:仮リンク]] (counterpart of [[w:en:Template:Interlanguage link]]) when the linked article is only available in other languages. A quick search of Main+仮リンク on jawiki shows 2k results.

@Jdlrobson: I don't really understand why this has been declined and why the "existing default behaviour makes sense for most wikis".

I came across this with Joseph Nation and saw the hatnote. I tapped on it, expecting it to take me to an extant article, since the link was blue. That wasn't the case, which means that the hatnote should be removed or modified per Wikipedia:Hatnote#Non-existent articles. However, without tapping on the link, I have no way of knowing that the article doesn't exist and the hatnote should be removed/modified. I think that this declination should be undone: this non-standard behavior is not desirable at all.

Or to put this another way, if the reply will be "change it on the English Wikipedia": why does this behavior make sense as the default when the general MediaWiki expectation is that red links are actually red?

@Jdlrobson: I don't really understand why this has been declined and why the "existing default behaviour makes sense for most wikis".

@Sdrqaz the hatnote template is a local template that lives on wiki at Module:Hatnote. WikimediaMessages provides opinionated styles that we recommend applying to all wikis to minimize work for smaller communities to adapt their content to be mobile and dark mode friendly.

There is an argument here to be made a hatnote "Not to be confused with Joe Nation (YouTuber)" shouldn't exist if "Joe Nation (YouTuber)" is not notable enough to have a Wikipedia article, but these are discussions to have in the community, not here on Phabricator.

By design communities are free (with consensus) to fork or remove these styles by following the guidelines https://www.mediawiki.org/wiki/Extension:WikimediaMessages#Applying_styles_only_to_specific_skins

If you feel strongly that red link behaviour is needed there, I suggest you make a topic of discussion on the talk page for https://en.m.wikipedia.org/w/index.php?title=Module:Hatnote and request for the template to be modified.
This should be as simple as adding

.hatnote a.new {
    color: var(--color-destructive, #bf3c2c) !important;
}

to https://en.m.wikipedia.org/w/index.php?title=Module:Hatnote/styles.css

Alternatively an interface admin can choose to fork the hatnote styles using the guidance in https://www.mediawiki.org/wiki/Extension:WikimediaMessages#Applying_styles_only_to_specific_skins

I hope this is helpful. Feel free to ping me on any talk page discussions if you decide to go down that route and have any questions.

@Jdlrobson-WMF: That's a disappointing response, especially because it isn't demonstrated why the behavior of not rendering a redlink in a hatnote is desirable, apart from saying that it "minimize[s] work for smaller communities to adapt their content to be mobile and dark mode friendly".

Unanswered is my previous question:

Or to put this another way, if the reply will be "change it on the English Wikipedia": why does this behavior make sense as the default when the general MediaWiki expectation is that red links are actually red?

The link color appears to have been added in T173600: Improve treatment of hatnotes without a single iota of public rationale. I think the approach at

In T216766#7482556, @Jdlrobson wrote:

If this was not intentional, the rule .hatnote a, .dablink a, .rellink a can be removed since that rule sets the hatnote link color to the default link color and is redundant. If it is intentional a comment should be added to the code.

Is the correct approach to resolving this task, not telling en.wiki they can work around this without supplying that rationale.

The best I can see for a plausible rationale is that the introduction of background color causes some issue with contrast accessibility that is an issue with the standard red link color but not the blue. If in fact that is the concern, I would suggest redlinks are a low priority for that concern in general, and would be generally supportive of simply removing the lines of interest.

Change #1117577 had a related patch set uploaded (by Saint Johann; author: Saint Johann):

[mediawiki/extensions/WikimediaMessages@master] Remove hatnote styles that disable red/visited link colours

https://gerrit.wikimedia.org/r/1117577