Small (or fine) prints and side comments on a page, such as disclaimers, copyrights, licensing information, and legal restrictions, should not be achieved by reducing the size of font, using CSS, as this gives them no semantic meaning, which means that they will not be recognized as such by user agents. In order to give them semantic meaning and therefore make the meaning accessible to more users, the <small> element, which makes text small, should be used. It is important to note that the element should not be used for main texts, such as multiple paragraphs and a whole page, as these are not side comments to a page; rather, it should be used for short runs of text. Figure 3.22 shows how it is used, and Figure 3.23 depicts the rendered result. Small text may also be marked up as important by using the <strong> element.
Why should you not reduce font size to achieve small text instead of using the <small> element?
Source: Sklar David (2016), HTML: A Gentle Introduction to the Web’s Most Popular Language, O’Reilly Media; 1st edition.