Somehow SE's MathJax doesn't like \DeclareMathOperator in subscripts or superscripts, this doesn't render on SE:

$\DeclareMathOperator{\two}{two}$ $\two+\two=\two^\two$ 

(I can't directly demonstrate the failure here, since MathJax seems not available in this community.)

But in the MathJax demo it renders just fine:

What's the reason? Different version of MathJax?