<thead>
The block of rows holding the column headers.
Baseline: Widely available · Tables
When to use it
One per table, before tbody; makes the header row(s) repeat on printed pages and sticky-positionable.
When not to
Header cells inside it still need scope='col' — thead alone doesn't associate them.
Example
<thead>
<tr><th scope='col'>Day</th><th scope='col'>Loaves</th></tr>
</thead>