MathJax Rendering Error: Commands Like `\oiint` Not Recognized

[blue_emperor]

Math Double-Closed-Path-Integral Symbol

actually, I now realise a much nicer solution is \newcommand{\oiint}{{\subset\!\supset} \mathllap{\iint}} you can even have an \oiiint that doesn’t look half bad \newcommand{\oiiint}{{\Large{\subset\!\supset}} \mathllap{\iiint}}