<legend>
The caption of a fieldset, announced with every control inside the group.
Baseline: Widely available · Forms
When to use it
First child of fieldset, naming the group ('Delivery details', 'Payment method').
When not to
Only valid as fieldset's first child; keep it short — screen readers repeat it per control.
Example
<fieldset>
<legend>Delivery details</legend>
…controls…
</fieldset>