silverstripe-mapboxfield/client/src/styles/bundle.scss

.mapbox__map-outer {
    position: relative;
    height: 0;
    padding-bottom: 56.25%;

    // 16x9
    border: 1px solid #ced5e1;
}

.mapbox__map-inner {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
}

.mapbox__map {
    width: 100%;
    height: 100%;
}