“Ruby” are short runs of text alongside the base text, typically used in East Asian documents to indicate pronunciation or to provide a short annotation. This specification defines markup for ruby, in the form of an XHTML module.

Here’s a sample…
Firefox’s ruby markup rendering:

Internet Explorer’s ruby markup rendering:

Here’s how it should look (ignoring the difference of font and font size):

So far Firefox (v1.5.0.7) is not rendering it correctly at all and strangely enough Internet Explorer (v6) shows the example correct but still has very little but at least some support. From the examples I tried myself, IE is OK as long as there is only one ruby text tag and one base tag. The main problem for me is that I don’t use IE on a everyday basis. I’ve seen ruby markup used in print (dictionaries) but the print world and web world can have vast underlining differences as pointed out in the article.

…the Web may lead to some phenomena and problems that are not present in traditional typography. Structural markup for ruby, as defined in this specification, cannot guarantee that ruby text will always be rendered alongside the base text. There are a very wide variety of current and future output devices for documents marked up with XHTML.

Anyone thinking that the web will cause print to be extinct soon shouldn’t hold their breath yet.

Update: There is now a .xpi extension available for XHTML Ruby Support by Hiroshi Shimoda