Looking at https://johnmacfarlane.net/babelmark2/?text=**a*_b**
There seems to be disagreement in the wild:
CommonMark wants **a*_b**
to render as
<p><em><em>a</em>_b</em>*</p>
Whereas lots of the established players render as:
<p><strong>a*_b</strong></p>
Does this double <em>
even make sense conceptually, it seems like some weird HTML to me.