I feel that no one can compete with Skia in terms of feature-richness. But performance, it'll really depend on the environment, hardware, and what you are doing.

Unity's UI Toolkit, for example, is mesh and shader-based, so it can be highly performant for complex UIs.

Interesting note: If you use OneJS with the NodeJS backend, you can use the wasm version of Skia and render all kinds of cool stuff onto a texture in Unity. Obviously the performance with that is not ideal for animation. But I was able to quickly and easily render music notations and charts on my deployed Unity app on Android using that method.

> Why not embedding a browser directly though? How do you feel about the Flutter based UI Widgets?

Mostly to avoid overheads on both disk space and runtime performance. QuickJS is only ~20MB to embed. And I have no experience with Flutter, unfortunately. =/