Running a cesium viewer within a div.

I'm just starting out with Cesium js.
I can host the HellowWorld.html on local host and see the globe and stars.

If I try to set up the a new Cesium.Viewer in a div on a page with a second div with control buttons, the Cesium viewer still takes over 100% of the browser window.

Does anyone have an example/advice of using a Cesium Viewer with other controls on a web page? It seems like this should be a common thing to want to do but I can't get it to display within the div.