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HTML 

              
                <header class="header" role="banner">
  <h1>Flexbox YouTube Thumbnail Grid</h1>
</header>
<main role="main">
  <article>
    <p>Responsive flexbox unordered list grid of YouTube video thumbnails with Fancybox links. Uses a 9kb image with a 16:9 aspect ratio to be able to use the YouTube thumbnails as a background image, without seeing those black letterbox bars at the top and bottom. This also makes the video thumbnails fluid when resizing the browser.</p>
  </article>
  <section class="videos" id="featured-videos">
    <div class="video-grid front-page" id="front-page-videos">
      <ul class="video-list featured">
        <li class="video featured">
          <a data-fancybox href="https://www.youtube.com/embed/Zq6Crtglztk?autoplay=1&controls=0&showinfo=0&color=white&rel=0" class="featured-video">
            <figure style="background-image: url(https://img.youtube.com/vi/Zq6Crtglztk/hqdefault.jpg);">
              <img src="https://s3-us-west-2.amazonaws.com/s.cdpn.io/50598/video-thumb-placeholder-16-9.png" />
              <figcaption>Swap-Meet Speed for Roadkill Nights</figcaption>
            </figure>
          </a>
        </li>
        <li class="video featured">
          <a data-fancybox href="https://www.youtube.com/embed/qDK86kzgUPg?autoplay=1&controls=0&showinfo=0&color=white&rel=0" class="featured-video">
            <figure style="background-image: url(https://img.youtube.com/vi/qDK86kzgUPg/hqdefault.jpg);">
              <img src="https://s3-us-west-2.amazonaws.com/s.cdpn.io/50598/video-thumb-placeholder-16-9.png" />
              <figcaption>Cummins Diesel in a Cadillac</figcaption>
            </figure>
          </a>
        </li>
        <li class="video featured">
          <a data-fancybox href="https://www.youtube.com/embed/uSB2_428P2w?autoplay=1&controls=0&showinfo=0&color=white&rel=0" class="featured-video">
            <figure style="background-image: url(https://img.youtube.com/vi/uSB2_428P2w/hqdefault.jpg);">
              <img src="https://s3-us-west-2.amazonaws.com/s.cdpn.io/50598/video-thumb-placeholder-16-9.png" />
              <figcaption>Dirt-Track Challenger Rescue and Thrash</figcaption>
            </figure>
          </a>
        </li>
        <li class="video featured">
          <a data-fancybox href="https://www.youtube.com/embed/pOhl6oVpEes?autoplay=1&controls=0&showinfo=0&color=white&rel=0" class="featured-video">
            <figure style="background-image: url(https://img.youtube.com/vi/pOhl6oVpEes/hqdefault.jpg);">
              <img src="https://s3-us-west-2.amazonaws.com/s.cdpn.io/50598/video-thumb-placeholder-16-9.png" />
              <figcaption>Junkyard 1956 Buick Hack</figcaption>
            </figure>
          </a>
        </li>
        <li class="video featured">
          <a data-fancybox href="https://www.youtube.com/embed/PVs_xZ46Cxw?autoplay=1&controls=0&showinfo=0&color=white&rel=0" class="featured-video">
            <figure style="background-image: url(https://img.youtube.com/vi/PVs_xZ46Cxw/hqdefault.jpg);">
              <img src="https://s3-us-west-2.amazonaws.com/s.cdpn.io/50598/video-thumb-placeholder-16-9.png" />
              <figcaption>Wheelstanding Dump Truck! Stubby Bob’s Comeback</figcaption>
            </figure>
          </a>
        </li>
        <li class="video featured">
          <a data-fancybox href="https://www.youtube.com/embed/C4jk51rgfts?autoplay=1&controls=0&showinfo=0&color=white&rel=0" class="featured-video">
            <figure style="background-image: url(https://img.youtube.com/vi/C4jk51rgfts/hqdefault.jpg);">
              <img src="https://s3-us-west-2.amazonaws.com/s.cdpn.io/50598/video-thumb-placeholder-16-9.png" />
              <figcaption>Will It Go 10s? Mazdarati Is Back!</figcaption>
            </figure>
          </a>
        </li>
      </ul>
    </div
  </section>
</main>
!

CSS 

              
                /**
 * Grid styles
 */
.video-grid.front-page {
  max-width: 1280px;
  margin: 0 auto;
  padding: 1em 2em;
}

ul.video-list {
  display: flexbox;
  display: flex;
  justify-content: center;
  flex-wrap: wrap;
}

li.video {
  flex-grow: 1;
  position: relative;
  ooverflow: hidden;
	width: 33.333333333%;
	width: -webkit-calc(100% / 3);
	width: calc(100% / 3);
  border: solid 0.2em transparent;
  
  & a {
    outline: none;
  }
      
  &:before {
    content: '';
    display: block;
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background-color: rgba(0,0,0,0);
    transition: background-color .15s;
  }
  
  & figure {
    display: block;
    position: relative;
    overflow: hidden;
    background-repeat: no-repeat;
    background-position: center center;
    background-size: cover;
    
    & img {
      display: block;
      max-width: 100%;
      height: auto;
      opacity: 0;
      transform: scale(0.5);
      transition: all .20s;
    }

    & figcaption {
      position: absolute;
      bottom: 0;
      width: 100%;
      background: rgba(0,0,0,0.6);
      color: #fff;
      font-size: 1.4rem;
      font-weight: 600;
      padding: 1rem;
      transform: translateY(0);
      opacity: 1;
      transition: all .20s;
    }
  }
  
  &:hover {
    & figure {
      & img {
        opacity: 1;
        transform: scale(1);
      }
      
      & figcaption {
        opacity: 0;
        transform: translateY(50%);
      }
    }
  }
  
  &:hover:before {
    background-color: rgba(0,0,0,0.3);
  }
}

@media screen and (max-width: 800px) {
	li.video {
		width: 50%;
		width: -webkit-calc(100% / 2);
		width: calc(100% / 2);
	}
}

@media screen and (max-width: 640px) {
	li.video {
		width: 100%;
	}
}

/**
 * Pen styles
 */
html { box-sizing: border-box; height: 100%; overflow-y: scroll; font-size: 62.5%; }
*, *:before, *:after { box-sizing: inherit; }
body {
	background: #fff;
	color: #444;
	font-family: 'Open Sans', Arial, -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen-Sans, Ubuntu, Cantarell, 'Helvetica Neue', sans-serif;
	font-size: 1.8rem;
	line-height: 1.2;
}

header {
  width: 100%;
  padding: 2em;
  box-shadow: 0px 0px 10px rgba(0,0,0,0.7);
  
  & h1 {
    font-size: 3rem;
    font-weight: 700;
    text-align: center;
    letter-spacing: -0.03em;
    color: #666;
  }
}

main {
  padding: 2rem 0;
}

article {
  max-width: 1280px;
  margin: 0 auto;
  padding: 1em 2em;
}
!

JS
!
999px

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