The HTML5 ruby element allows a word or phrase to be annotated with a small text above or below the original line.
The origin of ruby is in the Chinese and Japanese languages, where ruby annotations in simplified script or roman characters are being used as pronounciation guides for complex Chinese characters.
Currently (January 2010), ruby elements are supported in Internet Explorer, Google Chrome and in the latest Webkit development versions. Both Chrome and Webkit currently only support the HTML elements, but not yet the ruby CSS3 properties.
Outside of the traditional Japanese and Chinese use for ruby as pronounciation guide, a simple use case could be as part of language training. Roll over the German sentence below to see English translations for each word:
Dieser deutsche Satz hat englische Übersetzungen für alle Wörter.