The <rt> tag defines Character pronunciation of ruby annotations.
Ruby annotation is used to display certain characters in languages spoken in China, Hong Kong, Japan, Macau, Mongolia, North Korea, South Korea & Taiwan.
You must set the <rt> element inside the <ruby> element.
It is better to include the <rp> tag also as a fallback value. This will show the text inside this tag for all the browsers which does not support the <ruby> annotations.