Template:YouTube thumbnail/styles.css

.yt-thumb .embedvideo,
.yt-thumb .ev-container,
.yt-thumb iframe {
  width: 100% !important;     /* fill the 408px wrapper */
  max-width: none !important; /* don’t auto-shrink below the wrapper */
  height: auto;               /* keep aspect ratio unless height is fixed */
  display: block;
}