“Sliding Doors” is a “story” published by AO3 volunteers to demonstrate some of what you can do with normal CSS options available on the site. You can check out their tutorial for more, or look at some of the guides below. If you think that something is missing from one of these lists, then let me know!

• “Decorations for Fic,” by Jnsn, covers dividers, a chessboard whose pieces move when the board is hovered over, music, chapter titles, image outlines, drop caps, margin images, timetables, and news articles.
• “The Fic Writer's Guide to Formatting (An AO3 Repository),” by AnisaAnisa, covers HTML, links and images, page dividers, tables, borders, boxes, and customizations of fonts, links, titles, and usernames.
• “How to Override the Archive's Automated Chapter Headers,” by C Ryan Smith.
• “How to Use the newly available details and summary code,” by watcherless, shows you how to make clickable spoilers.
• “The Nice and Accurate Guide to Footnotes,” by Vigs, explains how to make simple footnotes, clickable footnotes that move you around the page, and mouseover footnotes that can be hovered over (or clicked, on mobile).
• “Ruby Text,” by Elsepth, lets you stack words in superscript above other words.
• “Skin for Putting Music Indicators in Your Fic,” by SpookyTesting.
• “Highlight comments of your favourite readers!,” by ElectricAlice.

La_Temperanza has a host of useful, general-purpose tutorials with self-explanatory titles:

• "How to Make Customized Page Dividers”
• “How to Make Invisible Text (That Can Be Highlighted)”
• “How to Change Text on AO3 When the Cursor is Hovering Over It (Or Clicked on Mobile”
• “How to Wrap Text around Images”
• “How to Make a ‘Choose Your Own Adventure' Fic”
• “How To Make Images Fit In Mobile Browsers”