WP Furigana enables you to add Furigana (a Japanese reading aid) using the visual editor. No knowledge of the ruby HTML tag is needed.
If you remove furigana in the visual editor, parts of the ruby tag will remain. It's very important not to leave empty ruby tags so the WordPress editor is cleaned up each time you add a new furigana. If you don't add any furigana after removing them in the visual editor, use the cleanup button introduced in the latest version of WP Furigana.
This plugin uses a custom CSS for Firefox, therefore your text's line-height is very important. Never give your text's line-height a value lower than the default (1).
Firefox is only supported at the front-end at the moment. The furigana has been colored blue though so it's possible to edit it in Firefox.
The rp tag is not supported in this version of WP Furigana.
Internet Explorer supports furigana and reading it works just fine, however, the plugin itself has not, and will not be tested in IE in a foreseeable future.