An answer today caught my attention: https://math.stackexchange.com/a/2185406/62967
Here's an image of what I see with Safari 10.0.3 (with Firefox 52.0 it's the same):
The link in the spoiler is not hidden. Here's the picture taken with the pointer hovering over the spoiler
Here's the relevant part of the source:
Can you explicitly write up the congruences?
>! See also [monogenic semigroups][1].
[1]: https://en.wikipedia.org/wiki/Monogenic_semigroup
A spoiler with a link follows. Can you see it?
Spoiler with link
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, but links are not under user's control. Anyway this is not related to MathJax. Not a big deal, but a bit annoying. $\endgroup$