I think for AI-generated websites, they tend to use a lot of emojis for almost everything. Also, they use colorful and bright colors. Other indication is using a single HTML file for a page, with embedded CSS and JS. In my opinion, most developers who wrotes the codes themselves most likely wouldn't heavily embed their CSS and JS into the HTML file because of readability.
Pioneer Space Sim, it offers unlimited space exploration in a huge procedural generated universe. It's like a mix of KSP and No Man's Sky.
0 A.D., it has a gameplay like AOE with a theme of the 0 A.D. periods.
Xonotic, a fast-paced online FPS game, but it looks like the server don't have much players recently.
The Dark Mod brings you an experience of being a thief and sneaking through the darkness. It has a lot of community-made maps.
If you'd like, I have an open source-game called Dungeon Maze (https://github.com/raihankr/dungeon-maze). It's a maze solving game with 2d pixel graphics. It was made with vanilla HTML, CSS, and JS. Utilizing the HTML canvas, I didn't use any game engine library in the development.