Lazy Loading background images in CSS

index.html

<div id="container">
  <h3>Lazy loading CSS background images</h3>
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Aliquam at ligula non lorem facilisis finibus. Suspendisse turpis lacus, tempus sit amet dignissim sit amet, pretium non sem. Ut malesuada purus ac magna placerat, tempus commodo turpis consectetur. Nunc vitae metus vehicula nunc congue varius. Suspendisse egestas urna non finibus feugiat. Vestibulum maximus, nunc et tempus convallis, dui ante vestibulum est, in lobortis purus sapien eget lacus. Suspendisse cursus enim non quam mollis imperdiet. Pellentesque convallis eget nunc vitae fringilla. Fusce interdum, velit semper varius aliquet, eros sapien facilisis erat, iaculis vulputate erat arcu vel neque. Suspendisse dignissim hendrerit lorem. Etiam metus ex, finibus vitae iaculis vitae, iaculis non ex. Phasellus commodo id ante id consequat. Nunc consectetur viverra lacus non fringilla. Pellentesque nec nisl vulputate, sagittis nisl id, venenatis nisi. Nunc malesuada tellus a elit suscipit tempor.
  </p>
  <p>
Nam lacinia tortor quis volutpat lacinia. Aliquam in orci in nunc vehicula maximus. Phasellus elementum nulla augue, at aliquam sem pulvinar dapibus. Vivamus molestie venenatis risus pulvinar interdum. Phasellus blandit tortor eget nulla sagittis auctor. Cras sed leo in velit lobortis euismod. Suspendisse non ante tellus.
  </p>
  <div id="bg-image" class="lazy"></div>
  <p>
Nam lacinia tortor quis volutpat lacinia. Aliquam in orci in nunc vehicula maximus. Phasellus elementum nulla augue, at aliquam sem pulvinar dapibus. Vivamus molestie venenatis risus pulvinar interdum. Phasellus blandit tortor eget nulla sagittis auctor. Cras sed leo in velit lobortis euismod. Suspendisse non ante tellus.
  </p>
</div>

lazy-loading-background-images-in-css.markdown

Lazy Loading background images in CSS

A Pen by ImageKit.io on CodePen.

License.

script.js

document.addEventListener("DOMContentLoaded", function() {
  var lazyloadImages;    

  if ("IntersectionObserver" in window) {
    lazyloadImages = document.querySelectorAll(".lazy");
    var imageObserver = new IntersectionObserver(function(entries, observer) {
      entries.forEach(function(entry) {
        if (entry.isIntersecting) {
          var image = entry.target;
          image.classList.remove("lazy");
          imageObserver.unobserve(image);
        }
      });
    });

    lazyloadImages.forEach(function(image) {
      imageObserver.observe(image);
    });
  } else {  
    var lazyloadThrottleTimeout;
    lazyloadImages = document.querySelectorAll(".lazy");
    
    function lazyload () {
      if(lazyloadThrottleTimeout) {
        clearTimeout(lazyloadThrottleTimeout);
      }    

      lazyloadThrottleTimeout = setTimeout(function() {
        var scrollTop = window.pageYOffset;
        lazyloadImages.forEach(function(img) {
            if(img.offsetTop < (window.innerHeight + scrollTop)) {
              img.src = img.dataset.src;
              img.classList.remove('lazy');
            }
        });
        if(lazyloadImages.length == 0) { 
          document.removeEventListener("scroll", lazyload);
          window.removeEventListener("resize", lazyload);
          window.removeEventListener("orientationChange", lazyload);
        }
      }, 20);
    }

    document.addEventListener("scroll", lazyload);
    window.addEventListener("resize", lazyload);
    window.addEventListener("orientationChange", lazyload);
  }
})

style.css

* {
  font-family: sans-serif;
}

#container {
  font-size: 20px;
  line-height: 30px;
  max-width: 600px;
}

#bg-image.lazy {
   background-image: none;
   background-color: #F1F1FA;
}
#bg-image {
  background-image: url("https://ik.imagekit.io/demo/img/image10.jpeg?tr=w-600,h-400");
  max-width: 600px;
  height: 400px;
}