<rp>
Fallback parentheses shown only by browsers that can't render ruby layout.
Baseline: Widely available · Inline text semantics
When to use it
Wrap the rt in a pair of rp elements containing '(' and ')' so the annotation degrades to '漢(kan)'.
When not to
Only valid inside ruby, flanking rt.
Example
漢
<ruby lang='ja'>漢<rp>(</rp><rt>kan</rt><rp>)</rp></ruby>