“HTML Living Standard: Text-Level Semantics: 4.5.10: The ruby Element”, 2020-01-29 ():
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…The ruby & rt elements can be used for a variety of kinds of annotations, including in particular (though by no means limited to) those described below. For more details on Japanese Ruby in particular, and how to render Ruby for Japanese, see Requirements for Japanese Text Layout.
…Note: At the time of writing, CSS does not yet provide a way to fully control the rendering of the HTML ruby element. It is hoped that CSS will be extended to support the styles described below in due course.
Example: Mono-ruby for individual base characters in Japanese: One or more hiragana or katakana characters (the ruby annotation) are placed with each ideographic character (the base text). This is used to provide readings of kanji characters:
<ruby>B<rt>annotation</ruby>