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

<style>
    loading-screen {
        display: flex;
        flex: 1;
        z-index: 100;
        justify-content: center;
        align-items: center;
        background-color: black;
    }

    viewer {
        display: none;
        position: relative;
        overflow: hidden;
        /* Start stage */
        flex: 1;
        z-index: 1;
        justify-content: center;
        align-items: center;
    }
</style>

<viewer></viewer>
<loading-screen></loading-screen>