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

<style>
    loading-screen {
        position: absolute;
        z-index: 100;
        opacity: 1;
        pointer-events: none;
        display: flex;
        justify-content: center;
        align-items: center;
        -webkit-transition: opacity 1.5s ease;
        -moz-transition: opacity 1.5s ease;
        transition: opacity 1.5s ease;
    }

    viewer {
        position: relative;
        overflow: hidden;
        /* Start stage */
        flex: 1;
        z-index: 1;
        justify-content: center;
        align-items: center;
    }
</style>

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