Icon sets, such as, Font Awesome, Glyphicons, or Google Fonts, commonly use the <i> tag.

One reason is that <i> is shorter than <span>.

Another reason is that the letter i can also stand for 'icon'.

Technically, either the <i> or the <span> tag can be used to render icons.

A twitter icon by Font Awesome created with the <i> tag.

<i class="fab fa-twitter"></i>