A ruby annotation:
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.
The <rt> tag is new in HTML5.
The <rt> tag also supports the Global Attributes in HTML.
The <rt> tag also supports the Event Attributes in HTML.
Most browsers will display the <rt> element with the following default values: