For what do you use HTML in notes? (new guy question)

I’m new to HTML and CSS–so new that I added my first custom CSS snippet tonight.

I learned that there is some interaction that happens between this file and the HTML in my notes. Specifically, this HTML I found allows for embedded YouTube iframes to be responsive.

Of course the ability to do this for a new guy is mind blowing. Now I’m super curious what other things you can do with CSS/HTML combos.

What do you use it for?