TransparentGlobe.js

//Create's a 1x1 transparent base layer so that the globe has no imagery
var transparentBaseLayer = new Cesium.SingleTileImageryProvider({
    url : "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mNgYAAAAAMAASsJTYQAAAAASUVORK5CYII="
});

//Create the viewer, must specify alpha :true in order for the globe and
//background to be transparent
var viewer = new Cesium.Viewer('cesiumContainer', {
    skyBox : false,
    skyAtmosphere : false,
    baseLayerPicker : false,
    imageryProvider : transparentBaseLayer,
    contextOptions : {
        webgl: {
            alpha: true
        }
    }
});

//Set the background of the scene to transparent
viewer.scene.backgroundColor = Cesium.Color.TRANSPARENT;

//Set the globe base color to transparent
viewer.scene.globe.baseColor = Cesium.Color.TRANSPARENT;

//Work around https://github.com/AnalyticalGraphicsInc/cesium/issues/2866
viewer.scene.fxaa = false;

var rectangle = Cesium.Rectangle.fromDegrees(-115.0, 37.0, -107, 40);
//Add our region limited imagery
viewer.imageryLayers.addImageryProvider(new Cesium.OpenStreetMapImageryProvider({
    url : '//a.tile.openstreetmap.org/',
    rectangle : rectangle
}));

viewer.camera.viewRectangle(rectangle);