<small>
Fine print: caveats, credits, legal notices.
It is not "smaller text", it is a side comment. The specification names contract footers, disclaimers, copyright notices and attributions exactly.
The reduced size is only the default style. The meaning is a secondary remark that is still part of the document.
<footer>
<small>
Rarity is the curators' opinion, not the specification's.
</small>
</footer>