A ruby annotation:
漢 <rt> ㄏㄢˋ </rt>
Definition and Usage
The <rt> tag defines an explanation or pronunciation of characters (for East Asian typography) in a ruby annotation.
Use the <rt> tag together with the <ruby> and 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 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 <rt> 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.