On some other SE sites, code in between dollar signs gets rendered as mathematics (using MathJaX, I believe). This doesn't seem to work here? Why not? And how do I get round it?
On this site, we want to see the actual code far more often than we want to see the rendered output, so MathJaX is not enabled for this site. If you want to show the result of some input, you need to create an image of the output and upload it. One of the simplest methods of getting an image from your code is to use the standalone package (see Compile a latex document into a png image that's as short as possible. for more details). To upload it, click on the "add image" button at the top of the text box (the box symbol next to the one with the 1s and 0s) and, if you have at least 10 reputation points, you will be able to upload the image and have it embedded in your question/answer.
Just as a follow-up to the exchange with Andrew in the other answer, this is what my fiddling vs. what I would have like looks on tex.SE compared to math.SX:
No, it's not dramatic that this doesn't work and I could help myself with simple markup and by digging for that $\times$ sign. My point is, that it is a simple convenience that is already implemented, whose availability doesn't hurt anybody and creating an actual TeX file for this, compiling it, converting it to a PNG file and uploading it is so overkill for this use case. And I'm not even sure how well inline images work here.
And concerning the HTML/unicode solution compared to the TeX syntax: the latter might not be rendered by TeX but it still looks much better than what I came up with. Yes, I might have fetched a real minus sign from some unicode table, too, but the more work I have to invest to make such a simple thing look decent, the stronger – I feel – my point becomes really.
Just as a convenience, here's my question again that sparked these posts: Omit zeros before the decimal point and convert scientific notation in siunitx