iframe[data-src]:not([src]) {
    background: linear-gradient(90deg, #f0f0f0 25%, #e0e0e0 50%, #f0f0f0 75%);
    background-size: 200% 100%;
    animation: loading 1.5s infinite;
    min-height: 400px;
}
@keyframes loading {
    0% { background-position: 200% 0; }
    100% { background-position: -200% 0; }
}