How come classes, <em> and <span> tags aren't really explained in the HTML course?

Question

How come classes, <em> and <span> tags aren't really explained in the HTML course?

Answer 1

<em> - Emphasized text
The <span> tag is used to group inline-elements in a document.
You can use https://www.w3schools.com that gives more tutorials.

Answer 2

The HTML course provides basic information. If you want to go deep you'll have to do research.
Besides, the elements mentioned in your question are pretty simple, they don't require extreme understanding.