From SPCodex, The Smashing Pumpkins wiki
.video {
padding: 3px;
}
.video-with-caption {
background-color: #f8f9fa;
border: 1px solid #c8ccd1;
font-size: 94%;
padding: 3px;
margin-bottom: 24px;
}
@media (max-width: 768px) {
.video {
width: 100% !important;
}
.video iframe {
width: 100% !important;
}
}