Using Cesium without the Viewer

I would like to test Cesium in an environment where i have only access to a gl rendering context and its rendering context. Right now i cant because the Viewer class is hardly bound to html.
My question is this, is it possible to use Cesium without the Viewer class? Is there any example of this?

To give more context i want to test Cesium inside Nativescript which is cross platform mobile framework.
In there we have a plugin which gives us a WebGL2RenderingContext context which we can pass to most webgl libs like Three / iTown / Deck.gl …
But in the case of Cesium as the Viewer create html GUI and is bound to knockout, it wont work.