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

<style>
    fill-container {
        display: flex;
    }
</style>

{{#unless disable}}
<style>
    fill-container {
        width: 100%;
        height: 100%;
        flex-direction: column;

        justify-content: flex-start;
        /* align items in Main Axis */
        align-items: stretch;
        /* align items in Cross Axis */
        align-content: stretch;
        /* Extra space in Cross Axis */
    }

    fill-container>* {
        flex: 1 1 auto;
        flex-direction: row;

        justify-content: flex-start;
        /* align items in Main Axis */
        align-items: stretch;
        /* align items in Cross Axis */
        align-content: stretch;
        /* Extra space in Cross Axis */
    }
</style>
{{/unless}}

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