/* Target the WordPress Block AND standard iframes */
.wp-block-embed-youtube, 
.wp-block-embed__wrapper,
.video-container {
    position: relative;
    width: 100%;
    padding-bottom: 56.25%; /* Fixed 16:9 Ratio */
    height: 0;
    overflow: hidden;
}

/* Make the video fill the container */
.wp-block-embed-youtube iframe,
.wp-block-embed__wrapper iframe,
.video-container iframe {
    position: absolute;
    top: 0;
    left: 0;
    width: 100% !important;
    height: 100% !important;
    border: 0;
}