Defer Offscreen Images

Deferring offscreen images is another approach to optimizing the loading of images that are not immediately visible when a user first visits your website. While lazy loading postpones the loading of images until they come into the viewport, deferring offscreen images delays their loading until after the main content of the page has been loaded. This can further improve the perceived performance of your website. Here's how to defer offscreen images:

1. Use the defer Attribute:

   • The defer attribute is used to indicate that a script (or in this case, an img element) should be executed or loaded after the main content has been parsed. Apply the defer attribute to your img elements to defer their loading.

   html
   <img src="image.jpg" alt="Description" defer>
   

2. Set the src Attribute:

   • Similar to lazy loading, make sure to provide a src attribute with a low-resolution or placeholder image to ensure that some content is displayed while the images are being deferred.

   html
   <img src="placeholder.jpg" data-src="image.jpg" alt="Description" defer>
   

3. Use JavaScript to Swap src Attributes:

   • Use JavaScript to detect when the page's main content has been loaded. Then, swap the data-src attribute to the src attribute of the img elements to initiate the image loading.

   javascript
   // Wait for the DOMContentLoaded event before swapping src attributes
   document.addEventListener('DOMContentLoaded', function() {
     const images = document.querySelectorAll('img[defer]');
     images.forEach(img => {
       img.src = img.dataset.src;
       img.removeAttribute('data-src');
     });
   });
   

4. Minimize JavaScript Execution:

   • The JavaScript snippet should be small and efficient to avoid any delay in the rendering of the rest of the page. Ensure that the JavaScript code does not introduce performance bottlenecks.

5. Test and Optimize:

   • Test your deferring offscreen images implementation across different devices and browsers. Monitor your website's performance to ensure that offscreen images are being deferred and loaded as intended.

6. Consider Critical Images:

   • While deferring offscreen images is effective for improving page load times, be cautious with critical images that are necessary for understanding the content or user interaction. These images should still be loaded in a timely manner.

Deferring offscreen images is a technique that complements other optimization strategies like lazy loading and proper image sizing. Depending on your website's design and content, you can choose the most suitable approach or even combine multiple techniques for optimal performance.