Building unicode symbols from available symbols - mathjax/MathJax-docs GitHub Wiki
parallel and equal x22D5
From https://groups.google.com/d/msg/mathjax-users/r7rO-PDQtqc/M-kkSjKbkQoJ
MathJax fonts don't include this symbol, but you could use \unicode{x22D5}
to obtain it, provided your viewer has a font that includes it (not guaranteed). You might also try
\mathrel{\rlap{\,\parallel}}=
as an alternative that uses only the MathJax fonts.