<ruby>
Annotates base text with small readings above it — furigana, pinyin, pronunciation guides.
Baseline: Widely available · Inline text semantics
When to use it
For East Asian pronunciation annotations; pair with rt for the reading and rp for fallback parentheses.
When not to
Not for tooltips or footnotes in Latin-script prose — it's a typographic convention, not a comment mechanism.
Example
東京 (Tokyo)
<ruby lang='ja'>東<rp>(</rp><rt>とう</rt><rp>)</rp>京<rp>(</rp><rt>きょう</rt><rp>)</rp></ruby> (Tokyo)