As far as completed games that do non standard things with the interface, so far there's
Koi Koi, which if you click around a bit is a card game done with pure html. Then there's a poker game
Cows vs Aliens done with a mix of site scripting and HTML.
Delve is a roguelike.
I've seen other examples of demos that do some pretty advanced stuff, the editor is theoretically very flexible with the way it displays HTML, and yes javascript is doable by tying it in with the site scripting somehow too.
@BradinDvorak and @chumm might be the most knowledgeable about using JS, I'll go see if I can dig them up on Discord for you. (If you go to Brad's profile btw, there's a site extension that allows you to activate dark mode from your profile as well as some other features.)