Lazy Loading HTML5 picture element

Working example of lazy loading images using the picture element and intersection observer tested in Chrome and Firefox. Safari doesn't support intersection observer so the images are loaded immediately, and IE11 doesn't support the element so we fallback to the default img

The media queries in the media attr are arbitrary and can be set to suit.

The width threshold set is 960px - try a reload above and below this width to see either the medium(-m) or large(-l) variation of the image being downloaded when the image is scrolled into the viewport.

Codepen

<!-- Load images above the fold normally -->
<picture>
  <source srcset="img/city-m.jpg" media="(max-width: 960px)">
  <source srcset="img/city-l.jpg" media="(min-width: 961px)">
  <img class="fade-in" src="img/city-l.jpg" alt="city"/>
</picture>

<picture>
  <source srcset="img/forest-m.jpg" media="(max-width: 960px)">
  <source srcset="img/forest-l.jpg" media="(min-width: 961px)">
  <img class="fade-in" src="img/forest-l.jpg" alt="forest"/>
</picture>

<!-- Lazy load images below the fold -->
<picture class="lazy">
  <source data-srcset="img/river-m.jpg" media="(max-width: 960px)">
  <source data-srcset="img/river-l.jpg" media="(min-width: 961px)">
  <img data-srcset="img/river-l.jpg" alt="river"/>
</picture>

<picture class="lazy">
  <source data-srcset="img/desert-m.jpg" media="(max-width: 960px)">
  <source data-srcset="img/desert-l.jpg" media="(min-width: 961px)">
  <img data-srcset="img/desert-l.jpg" alt="desert"/>
</picture>

and the JS:

    document.addEventListener("DOMContentLoaded", function(event) {
   var lazyImages =[].slice.call(
    document.querySelectorAll(".lazy > source")
   )

   if ("IntersectionObserver" in window) {
      let lazyImageObserver = 
       new IntersectionObserver(function(entries, observer) {
          entries.forEach(function(entry) {
           if (entry.isIntersecting) {      
              let lazyImage = entry.target;
              lazyImage.srcset = lazyImage.dataset.srcset;
              lazyImage.nextElementSibling.srcset = lazyImage.dataset.srcset;
              lazyImage.nextElementSibling.classList.add('fade-in');
              lazyImage.parentElement.classList.remove("lazy");
             lazyImageObserver.unobserve(lazyImage);
            }
         });
        });

      lazyImages.forEach(function(lazyImage) {
       lazyImageObserver.observe(lazyImage);
      });
   } else {
     // Not supported, load all images immediately
    lazyImages.forEach(function(image){
        image.nextElementSibling.src = image.nextElementSibling.dataset.srcset;
      });
    }
  });

One last thought is that if you change the screen width back and forth, the image files are repeatedly downloaded again. If I could tie the above method in to a cache check then this would be golden...

For anyone still interested... After revisiting this issue, I came across a fairly new script called, Lazysizes. It's actually quite versatile, but more importantly it allows me to do lazy loading of images while utilizing the HTML5 markup as described in the OP.

Much thanks to the creator of this script, @aFarkas.

On web.dev Google advices that picture tags can be loaded lazy.
https://web.dev/browser-level-image-lazy-loading/

Images that are defined using the element can also be lazy-loaded:

<picture>
  <source media="(min-width: 800px)" srcset="large.jpg 1x, larger.jpg 2x">
  <img src="photo.jpg" loading="lazy">
</picture>

Although a browser will decide which image to load from any of the elements, the loading attribute only needs to be included to the fallback element.