A ruby annotation:
The <rp> tag defines what to show if a browser does NOT support ruby annotations.
Ruby annotations are used for East Asian typography, to show the pronunciation of East Asian characters.
Use the <rp> tag together with the <ruby> and the <rt> 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 <rp> tag is new in HTML5.
The <rp> tag also supports the Global Attributes in HTML.
The <rp> tag also supports the Event Attributes in HTML.