It's JavaScript (well, mostly TypeScript these days), but not much HTML (basically just a single canvas element :D). It's my own custom engine (used for a few commercial projects, and lots o' jam games), but it's all WebGL and built like a native engine (I don't even use HTML/CSS for fonts and stuff, I do that all in WebGL...). I might be up for trying a jam as a team, you can reach out to me at jimb.ly on Discord if that works for ya.