Currently, Moodle renders fontawesome icons using the <i> tag. Although the fontawesome docs do offer that option, the preferred, semantically correct way is to use a <span> tag.
The Fontawesome docs say:
"Font Awesome is designed to be used with inline elements, and we recommend sticking with a consistent HTML element to reference them by in your project. We like the <i> tag for brevity and because most folks use <em></em> for emphasized/italicized semantic text these days. If that’s not your cup of tea, using a <span> is more semantically correct."
We should really be using the <span> tag to be correct. The <i> tag is still a valid tag, regardless of the use of <em>.