.advert-slot--preview {
    margin: 0 0 27px;
    padding: 10px;
    border: solid 1px green;
    background-color: #e2f2e5;
}

.advert-slot--preview:before {
    display: inline;
    content: 'Advert Slot Selector: #';
}

    .advert-slot > * {
        display: block;
        margin-bottom: 21px;
    }