It's not a subset, it's the full WebKit layout engine underneath and supports most modern HTML5/CSS/JS.

Ultralight can render full websites just fine and is also a full browser runtime, the SDK ships with a Browser sample on macOS and Windows.

With that said, Ultralight is indeed somewhere between libRocket and Electron in terms of size and support. We won't support things that are unrelated to HTML UI (for example WebGL) and are able to reduce our dependencies/size/complexity by limiting said support.