Hey @ao3commentoftheday and anybody else who might know, is there a way to use different fonts in the body of a work on AO3 (and how much HTML do you have to know to make it work)?

You can, and you don't need to know any HTML to do it.

Use this AO3 public work skin. It's called Basic Formatting, and it has more than enough CSS to give you the font changes you're looking for, as well as a bunch of other things.

A work skin changes the appearance of your work on the Archive to everyone who accesses that work. This is different from a site skin, which changes the look of the site for the user who has that skin applied to their account.

You can add a work skin to your AO3 work in the same form where you enter the title and the tags and the text of the fic. Right below where you select your language, there's a spot for you to add a work skin. The three public ones provided by AO3 are already in that drop down. If you ever create a work skin of your own (or copy someone else's code) it'll be listed there too with whatever title you give it.

Once you've added the work skin to your work, you'll need to apply it onto the text that you want to modify. If you want it to modify the whole thing, that's just as easy as having it modify a single line. The steps are the same, it's all down to how many times you use them.

Adding in a cut because this got long

I updated the instructions under the cut to show how to modify text inside of a paragraph. Sorry for the confusion!