Note: Support for the footer was added in bug 66931 and https://gerrit.wikimedia.org/r/158605

The reason I chose #aaa as the border color in the mockup suggested there, is that it matches the #aaa currently used in: Infoboxes, all Mboxes, the ToC border, the category box border, and the H1/H2 underline.
cf. https://en.wikipedia.org/wiki/MediaWiki:Common.css etc

One option, would be to add a background color to the box. The standard is #F9F9F9. (I'm not sure why I didn't use that in my mockup?!). I'd recommend adding that.

#aaa does tend to work a lot better when the background colour is also changed, yeah. By itself it stands out (on white) as much too harsh and dark.

So yeah, do thatn but is there seriously no common class that the other things use? Why the hell not?

Bastardly elements.

Why can't we just remove the border and make the text in italic? That's the MediaWiki core default for remote repositories (example).

In T72629#1003979, @Nemo_bis wrote:

Why can't we just remove the border and make the text in italic? That's the MediaWiki core default for remote repositories (example).

There is not a single <i> or <em> on that page (aside from the _Unicode_ tagline transcluded from Commons).

The default message for this sort of thing in MediaWiki has no styling. E.g.

This file is from Wikimedia Commons and may be used by other projects. The description on its file description page there is shown below.

https://www.mediawiki.org/wiki/File:Example.svg
https://github.com/wikimedia/mediawiki/blob/3240b71f/languages/i18n/en.json#L1462-L1466
https://github.com/wikimedia/mediawiki/blob/dd8c1e2dd/includes/page/ImagePage.php#L646-L649

Classes available in MediaWiki suitable for this purpose:

• toccolours
• mw-warning

Either seem like a good fit. mw-warning seems a little better as it has additional left/right margin and centres the text.

In T72629#1044065, @Krinkle wrote:

There is not a single <i> or <em> on that page (aside from the _Unicode_ tagline transcluded from Commons).

.sharedUploadNotice {
    font-style: italic;
}

In T72629#1044680, @Nemo_bis wrote:

In T72629#1044065, @Krinkle wrote:

There is not a single <i> or <em> on that page (aside from the _Unicode_ tagline transcluded from Commons).

.sharedUploadNotice {
    font-style: italic;
}

Only in oldshared.css (legacy, used by CologneBlue and Nostalgia). Not part of shared.css, Vector, or MonoBook :)

@violetto @Jaredzimmerman-WMF @Prtksxna please help evolve the standard recommended styles for these types of in-page notices and ensure they're part of our style guide, thanks.

In T72629#1044849, @Krinkle wrote:

Only in oldshared.css (legacy, used by CologneBlue and Nostalgia). Not part of shared.css, Vector, or MonoBook :)

I don't know of what release you're talking (the wiki was upgraded few days ago), but in monobook the file description definitely has italic as of 1.24. I can attach a screenshot if needed. :) https://commons.wikimedia.org/wiki/File:InstantCommons_1.24_monobook.png