Wondering if there is any screenreader or skin that puts the message next to or below the form?
(Given the order of elements in the raw HTML without any CSS rendering, I'm not convinced that changing the reference makes sense, if there are no bottom-to-up human languages.)

I have seen sideways movements of such blocks (strangely on mobile displays - not even in landscape mode but there, too) and I do not know specifically on this one. Still, it is usually safer, not to use such references.

There is no bottom up in HTML/CSS currently, but rest assured, it is going to come with bustrophedon East Asian scripts sooner or later.

Change 320116 had a related patch set uploaded (by Nemo bis):
Remove redundant "below" from authmanager-create-from-login

https://gerrit.wikimedia.org/r/320116

Change 320116 merged by jenkins-bot:
Remove redundant "below" from authmanager-create-from-login

https://gerrit.wikimedia.org/r/320116