MediaWiki:Mobile.css

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/* All CSS here will be loaded for users of the mobile site */

/* Standard <youtube> (no lazy-load) */
.mw-parser-output iframe[data-extension="youtube"] {
  width: 100% !important;
  height: auto !important;
  aspect-ratio: 16 / 9;
  display: block;
}

/* Lazy-load wrapper + its thumbnail image */
.mw-parser-output .ext-YouTube-video {
  width: 100% !important;   /* override inline width="560px" etc. */
  height: auto !important;  /* override inline height */
}
.mw-parser-output .ext-YouTube-video img {
  width: 100% !important;   /* scale preview to screen */
  height: auto !important;
  aspect-ratio: 16 / 9;
  display: block;
}

/* Optional: cap on very large screens (adjust 720px to taste) */
@media (min-width: 992px) {
  .mw-parser-output iframe[data-extension="youtube"],
  .mw-parser-output .ext-YouTube-video {
    max-width: 720px;
    margin: 0 auto;
  }
}

.yt-thumb { max-width: 320px; margin:0.5em; }
.yt-thumb iframe,
.yt-thumb .ext-YouTube-video,
.yt-thumb img { width:100% !important; height:auto !important; aspect-ratio:16/9; display:block; }