<meta name="viewport" content="width=device-width, initial-scale=1" />
<style>
    body,
    model-viewer {
        width: 100%;
        height: 100%;
        margin: 0;
        padding: 0;
    }

    /* other-css */
</style>
<script type="module" src="model-viewer.min.js" defer></script>
<!-- body -->