If you don't know what the HTML5 ruby element is, you might want to take a minute to first read the section about the ruby element in the HTML5 specification and/or the Wikipedia article on ruby characters. To quote from the HTML5 description of the ruby element:
The ruby element allows one or more spans of phrasing content to be marked with ruby annotations. Ruby annotations are short runs of text presented alongside base text, primarily used in East Asian typography as a guide for pronunciation or to include other annotations. In Japanese, this form of typography is also known as furigana.
I give a specific example further down, but for now I want to first say that the really great news about the ruby element is that last week, Google Chrome developer Roland Steiner checked in a change (r50495, and see also related bug 28420) that adds ruby support to the trunk of the WebKit source repository, thus making the ruby feature available in WebKit nightlies and Chrome dev-channel releases.
A simple example
The following is a simple example of what you can do with the ruby element; make sure to view it in a recent WebKit nightly or Chrome dev-channel release. Note that the text is an excerpt from the source of a ruby-annotated online copy of the short story Run, Melos, Run by the writer Osamu Dazai, which I came across by way of Piro's info page for his XHTML Ruby add-on for Firefox (and which I mention a bit more about further below).
きのうの豪雨で山の水源地は<ruby>氾濫<rp>（</rp> <rt>はんらん</rt><rp>）</rp></ruby>し、濁流 <ruby>滔々<rp>（</rp><rt>とうとう</rt><rp>）</rp> </ruby>と下流に 集り、猛勢一挙に橋を破壊し、どうどうと 響きをあげる激流が、<ruby>木葉微塵<rp>（</rp> <rt>こっぱみじん</rt><rp>）</rp></ruby>に<ruby>橋桁 <rp>（</rp><rt>はしげた</rt><rp>）</rp></ruby> を跳ね飛ばしていた。
If you don't happen to have Japanese fonts installed, here's a screenshot of the source for reference:
Notice that the actual annotative ruby text (which I've highlighted in yellow in the source just for the sake of emphasis) is marked up using the rt element as a child of the ruby element, and the text being annotated is the node that's a previous sibling to that rt content as a child of the ruby element. The final new element in the mix is the rp element, which is simply a way to mark up the annotative ruby text with parenthesis, for graceful fallback in browsers that don't support ruby.
So here's the rendered view of that same text:
And here is a screenshot of how it should look in a recent WebKit nightly or Chrome dev-channel release:
Notice that the annotative ruby text is displayed above the ruby base it annotates. If you instead view this page in a browser that doesn't support the ruby feature, you'll see that the ruby text is just shown inline, in parenthesis following the ruby base it annotates. So the feature falls back gracefully in older browsers.
Support in other browsers
Current versions of Microsoft Internet Explorer also have native support for ruby, and you can also get ruby support in Firefox by installing Piro's XHTML Ruby add-on (and for more details, see his XHTML ruby add-on info page) — so we are well on the way to seeing the HTML5 ruby feature supported across a range of browsers. If you're not accustomed to reading printed books and magazines and such in Japanese, that might not sound like such a big deal. But for authors and developers and content providers in Japan who want to finally be able to use on the Web this very common feature of Japanese page layout from the print world, getting ruby support into another major browser engine is a huge win, and something to be very excited about.