In response to this question, Is there any difference between 旅行者 and 旅人?, the answerer used ruby for the two pertinent words. I was delightfully surprised to find it's accomplished with simple, self-explanatory markdown:
Japanese SE ignores attempts to do ruby with HTML:
I can't imagine why I would need ruby on Math SE or other Stack Overflow sites, but it would certainly be nice if I happened to need it and it was available. Or it could even be, for all I know, a standard feature of markdown.
So, what is the availability of markdown ruby outside of Japanese SE?