Our UI system isn't a HTML renderer. It's not using webkit.
It's a panel based system, like vgui, or panorama, or derma. It's coded in c#. It uses Yoga as a layout engine.
We style it using a css engine that we coded in c#. It takes in scss.
We have a Razor type system which allows you to mix html and c# to create easy, component based fully dynamic UI - quickly and iteratively. But you don't need to use that - you can create panels using new Panel() and create new panel classes without using any of this.