HTML <ruby> tag

HTML has a tag named <ruby> that allows you to add extra characters to the main text. This is useful for annotating the pronunciation of Kanji.

According to https://www.w3schools.com/tags/tag_ruby.asp:

A ruby annotation is a small extra text, attached to the main text to indicate the pronunciation or meaning of the corresponding characters.

<ruby>読<rt>よ</rt>む</ruby>

will look like this:

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