A ruby annotation:
漢 <rt> ㄏㄢˋ </rt>
Definition and Usage
The <ruby> tag specifies a ruby annotation.
A ruby annotation is a small extra text, attached to the main text to indicate the pronunciation or meaning of the corresponding characters. This kind of annotation is often used in Japanese publications.
Use the <ruby> tag together with the <rt> and/or the <rp> tags: The <ruby> element consists of one or more characters that needs an explanation/pronunciation, and an <rt> element that gives that information, and an optional <rp> element that defines what to show for browsers that do not support ruby annotations.
The numbers in the table specify the first browser version that fully supports the element.
Differences Between HTML 4.01 and HTML5
The <ruby> tag is new in HTML5.
The <main> tag also supports the Global Attributes in HTML.
The <main> tag also supports the Event Attributes in HTML.