Babylon.js/Viewer/assets/templates/default/defaultViewer.html

<style>
    .babylonjs-canvas {
        flex: 1;
        width: 100%;
        height: 100%;
    }

    nav-bar {
        position: absolute;
        height: 80px;
        width: 100%;
        bottom: 0;
        opacity: 0.3;
        background-color: black;
        color: white;
        transition: 1s;

        align-items: center;
        justify-content: center;
        display: flex;
    }
</style>

<canvas class="babylonjs-canvas" id="{{canvasId}}">
</canvas>

<nav-bar></nav-bar>