> Ruby, a form of interlinear annotation, are short runs of text alongside the base text. They are typically used in East Asian documents to indicate pronunciation or to provide a short annotation.

> This specification revises and extends the markup model established by HTML to express ruby.

The posted link is a short announcement. Here are the actual specs:

https://www.w3.org/TR/2024/WD-html-ruby-extensions-20240507/