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              <div class="container">

  <h1>Placeholders for images with known aspect ratios</h1>

  <p>These images do not have <code>width</code> and <code>height</code> attributes, but they still take up the right amount of space before they load. They don't push text down the screen as the browser figures out what size they are. This is very helpful
    for lazy loading images of unknown size with a known aspect ratio without disturbing the user as they read the page, possibly ahead of the lazy loading image.</p>

  <button>Load images</button>

  <p>This uses the same trick as responsive YouTube embeds. Each image is wrapped in a <code>div</code> element which uses a <code>padding-bottom</code> set to a percentage that matches the needed aspect ratio. The <code>img</code> element inside the div
    is set to take up 100% of its container's height and width. This way, the wrapping <code>div</code> is responsible for maitaining the height of the image, not the image itself.</p>

  <h2>Square</h2>

  <div class="fixed-aspect-ratio-container square">
    <img class="lazy" data-src="https://lorempixel.com/600/600/city" alt="square image" />
  </div>

  <h2>Landscape</h2>
  <div class="fixed-aspect-ratio-container landscape">
    <img class="lazy" data-src="https://lorempixel.com/600/337/city" alt="landscape image" />
  </div>

  <h2>Portrait</h2>
  <div class="fixed-aspect-ratio-container portrait">
    <img class="lazy" data-src="https://lorempixel.com/337/600/city" alt="portrait image" />
  </div>

</div>
            
          
!
            
              body {
  font-family:sans-serif;
  line-height:1.4;
}

h1, h2 {
  font-weight:normal;
}

.container {
  max-width:600px;
  margin:auto;
}
.fixed-aspect-ratio-container {
  position:relative;
  height:0;
  outline:1px solid #888;
  
  &.square {
    padding-bottom:100%;
  }
  
  &.landscape {
    padding-bottom:56.25%;
  }  
  
  &.portrait {
    padding-bottom:177%;
  }  

  > * {
    position:absolute;
    top:0;
    left:0;
    height:100%;
    width:100%;
  }
  
}

button {
  font-size:1.25em;
  padding:0.25em;
  border-radius:5px;
}
            
          
!
            
              document.querySelector("button").addEventListener("click", function(event) {

  // find lazy loaded things (image & iframe come to mind first)
  const images = document.querySelectorAll(".lazy[data-src]");

  // update their src
  Array.prototype.map.call(images, function(image) {
    image.src = image.dataset.src;
  });
});

            
          
!
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