Cool HTML learning resources?

This is a bit of a generic question but I’ve been really looking forward to spicing up some of my comments and even my fics with more than rich text. You can see some people pull off some crazy stuff on TikTok for example. It’s admittedly also been awhile since I’ve personally coded too so I’m not sure if there are any particular constraints AO3 has when inputting HTML into it yk?

I’m thinking of reopening my oooold SoloLearn account for this and CSS—but anything else you’d recommend helps! 🙏