ruby elementrt element, or an rp element, an rt element, and another rp element.HTMLElement.The ruby element allows one or more spans of
  phrasing content to be marked with ruby annotations. Ruby
  annotations are short runs of text presented alongside base text,
  primarily used in East Asian typography as a guide for
  pronunciation or to include other annotations. In Japanese, this
  form of typography is also known as furigana.
A ruby element represents the spans of
  phrasing content it contains, ignoring all the child rt
  and rp elements and their descendants. Those spans of
  phrasing content have associated annotations created using the
  rt element.
In this example, each ideograph in the Japanese text 漢字 is annotated with its reading in hiragana.
...
<ruby>漢<rt>かん</rt>字<rt>じ</rt></ruby>
...
   This might be rendered as:

In this example, each ideograph in the traditional Chinese text 漢字 is annotated with its bopomofo reading.
<ruby>漢<rt>ㄏㄢˋ</rt>字<rt>ㄗˋ</rt></ruby>
   This might be rendered as:

In this example, each ideograph in the simplified Chinese text 汉字 is annotated with its pinyin reading.
...<ruby>汉<rt>hàn</rt>字<rt>zì</rt></ruby>...
   This might be rendered as:
