Why? Linking to global user pages should be treated the same as linking to local user pages.

IMHO It's like links to disambiguations or redirects—most don't care where the link goes but there are a few who do care. For me personally in this case it'd be kinda indicator of whether the page is worth looking at. If I work in a wiki and see contribution by a user it's good to know from a first glance whether it's some red-paged noob, some random "global" guy with globaluserpage or it's a contributor who is experienced enough and cares enough to create a local userpage on which he might have provided some info about how he can be helpful in that wiki or whatever else wiki specific thing. Such a class or whatever marking would save some time which otherwise would be spent on opening the page and seeing the footer.

It seems that it is slightly similar to the problem of thing written in GlobalUserPage.php's @todo about wanted pages list

@Base do you use navpopups? (That's my usual method for preliminary info on
a username I don't recognize). I wonder if info could somehow be added to
what that pulls up, to solve this use-case?

Agree with @Quiddity that this kind of quick check is better served through previews/popup type functionality than color but added design tag so others can weigh in.

As the task description suggest I am also for a CSS-class added to links, so each user or wiki can create their own styling if they want.
It doesn't have to have a associated style by default as the Extension can be used by various wikis with each a different environment.

In T90015#1052230, @Base wrote:

IMHO It's like links to disambiguations or redirects—most don't care where the link goes but there are a few who do care. For me personally in this case it'd be kinda indicator of whether the page is worth looking at. If I work in a wiki and see contribution by a user it's good to know from a first glance whether it's some red-paged noob, some random "global" guy with globaluserpage or it's a contributor who is experienced enough and cares enough to create a local userpage on which he might have provided some info about how he can be helpful in that wiki or whatever else wiki specific thing. Such a class or whatever marking would save some time which otherwise would be spent on opening the page and seeing the footer.

+1 to this usecase in wikipedia.

Change 210001 had a related patch set uploaded (by Glaisher):
Mark links to global users pages with 'mw-globaluserpage' class

https://gerrit.wikimedia.org/r/210001

Change 210001 abandoned by Glaisher:
Mark links to global users pages with 'mw-globaluserpage' class

https://gerrit.wikimedia.org/r/210001