It's set to fade in from 0 opacity, for some sort of unnecessary "ooh look it's fancy" effect. My guess is that if you check the console you'll find that it hit some exception before it completed its little fade-in effect.
It seems to be caused by the FediAct addon, which doesn't make any sense. Disabling the addon fixes the page without needing a reload, so it must be a CSS issue rather than an exception. Edit: it seems the addon is overriding the fadeIn keyframe, but it looks fixed in the github version.
Might be worth reaching out to the addon authors. Hard to say whether the page or the addon is at fault, but they might be interested to know it even if it's the page's fault.
It looks like it's the addon's fault, and has already been fixed in the github version. It's also been abandoned, so it's probably not worth keeping around anyway.
I think that's the overlap between the dense region of PINs that start with 11, 12, or 13 (similar to the dense regions that start with 21, 22, 23, 31, etc), and the dense square region of month+day dates.
I got a PIN assigned by my bank back in the 1980s, and it is in that range. I always assumed it was random, because how easy is it to generate a 4-digit random number? But maybe they gave out PINs more like safe combinations. I don't think you could change them back then, either.
