A few of documents related to ruby.

Ruby is a name for small annotations that are rendered alongside base text. This is especially useful for Japanese and other East Asian content (ruby may be called furigana in Japanese).

HTML provides a way to mark up ruby text, and CSS can be used to fine tune the positioning of ruby text.

Rules for Simple Placement of Japanese Ruby
A simple set of rules for placement of Ruby text in Japanese typography.
The Case For Modern Ruby
Making the case for the need to go beyond ruby as implemented by Blink and Webkit in order to properly support the Japanese language and the needs of the Japanese publishing industry.
Better ruby in HTML
A rendering of a pull request against the HTML specification to restore the rb and rtc elements and update ruby content model accordingly.