In the long run, we should probably fix this. (If memory serves, Anton can add a few lines to MO's MathJax header material to include \mathit, although really it should be done at the whole MathJax level internet-wide.) I would particularly like \textsc.

In the particular case, however, in your comment and answer you correctly use \operatorname rather than \mathrm or \mathit or \text or whatever else people think to use for operator names. It gets the spacing right, and is supported by MathJax. (Moreover, I don't myself care for italicized words in math, so I think no matter what André was wrong to want \mathit in this case. If he really wanted an italicized operator name, I guess \operatorname{\it Diff} would have been correct, although I don't actually remember if MO's MathJax recognizes \it.)

In the very short run, MathJax does accomodate pretty-much arbitrary macroing. So you can always throw in a \newcommand\mathit{...} if you want to write something yourself; it will apply to the rest of that page.

I haven't been able to get any text to show up in italic, whatever I try. Maybe this one should be in Roman, but there should be some way to get italic text...