<sub>
Subscript text with typographic meaning.
Baseline: Widely available · Inline text semantics
When to use it
Chemistry (H₂O), math indices, footnote conventions that require subscripts.
When not to
Not for styling small lowered text — only where subscript position is part of the notation.
Example
Water is H2O.
<p>Water is H<sub>2</sub>O.</p>