Every once in a while, I find myself making edits like this one, where OP (likely due to inexperience in asking questions) has used display math mode (
$$ delimiters) and/or the
\dfrac commands in ways discouraged by the community.
This issue has been around for a while, see e.g. this meta question.
In line with the more-or-less universally supported initiative to be more communicative to especially new users, I have been making remarks similar to the one I just added to the comment templates thread, giving a short explanation of why not to use these functionalities of MathJax in titles.
In an attempt to make this all more accessible to new users without them having to commit one of these mistakes, this question intends to gather all guidelines on the use of MathJax in question titles in one place, the CW answer to this question.