Furigana is extremely useful if not indispensable in site dedicated to studying Japanese, but unfortunately the Stack Exchange software doesn't seem to support it out of the box. I've already seen here some questions and answers which could use some Furigana for clarification. You could always add the pronunciation inside parentheses, but that makes the text much more cumbersome in most cases.
HTML already supports Furigana through ruby tags, such as the following:
<ruby><rb>日本語</rb><rp>(</rp><rt>にほんご</rt><rp>)</rp></ruby>
This syntax is obviously not part of the very limited HTML subset supported by Stack Exchange, but perhaps it could be made part of it, at least for this site. Alternatively, some kind of Markdown syntax can be created for it, for instance:
[日本語]{にほんご}
could generate the verbose html code above.
rb
element). Perhaps[日本語]{にほんご}
?