I copied the code from this answer and pasted it into the question to which I linked. I don't know whether that changed anything or not, but it still doesn't work.

It looks as though it works if you set it off by line breaks, as in my suggested edit.

It's working now, although nothing changed as far as I know.

clemens‭ ok, Now I see what you meant by "set it off by line breaks."

Although I personally prefer having a double line break both before and after the MathJax section, for readability, only the double line break before is required for correct rendering.

This is not specific to MathJax. Mixing HTML and Markdown has a similar requirement: Details tag requires an internal blank line to render markdown

trichoplax‭ Interesting; so both HTML and Markdown formatting works, it seems, but not putting the two on the same line?

Then why does this work, I wonder: test HTML italics test Markdown italics?

I don't know the internal workings, but as I understand the consequences: HTML works inside Markdown, but Markdown doesn't work inside HTML (without a blank line to let the Markdown renderer know it's time to work again).

Using HTML and Markdown italics side by side (no overlap) is not a problem.

Nesting Markdown bold inside HTML italics seems to work on a single line, but not if the italics tags are on a separate line (unless a blank line is used).

<i>Some HTML italics containing **Markdown bold** part way through</i>

gives

Some HTML italics containing Markdown bold part way through

<i>
Some HTML italics containing **Markdown bold** part way through
</i>

gives

<i>

Some HTML italics containing **Markdown bold** part way through
</i>

gives

Some HTML italics containing Markdown bold part way through

(although this now seems to be ignoring the HTML italics).