Ruby is the name given to the small annotations in Japanese and Chinese content that are rendered alongside base text, usually to provide phonetic information, but sometimes to provide other information.
This article discusses how to use HTML5 markup for ruby text. It covers what works and what is still aspirational pending more widespread browser support.
The aim of markup is principally to establish the relationships between the base text and the ruby text (the annotations). Information about how to then apply adjustments to the default styling of ruby text which be covered by Ruby Styling, which is still in development.
Source: Web Design