Lazy Loading Best Practices for 2025

Lazy loading is one of those features that's almost trivial to turn on and surprisingly easy to get wrong. Done right, it trims your initial page weight without the visitor ever noticing. Done wrong, it delays your hero image or makes the page jump around as you scroll. Here's how to get the upside without the side effects.

1. Native is King (Usually)

You no longer need a jQuery plugin or any third-party script to defer images. The native loading="lazy" attribute works in every major browser, and it's smarter than the old scroll-listener hacks: the browser picks when to fetch each image based on scroll position, connection speed, and device, all without you writing a line of JavaScript.

<!-- Native, zero-dependency lazy loading -->
    <img 
        src="product-image.webp" 
        alt="Blue running shoes"
        width="600" 
        height="400" 
        loading="lazy" 
    />

2. Preventing Cumulative Layout Shift (CLS)

If you defer an image's load time, the browser does not know how much space to allocate for that image in the document flow. When the user scrolls down and the image finally renders, it pushes all the text and elements below it down the page. This is a Cumulative Layout Shift (CLS).

To prevent this, you must provide explicit width and height attributes. Modern browsers are smart enough to look at those attributes and calculate the CSS aspect-ratio on the fly, reserving a perfectly sized transparent box while the image loads over the network.

3. Background Images and Vanilla JavaScript

The native loading="lazy" attribute only works on <img> and <iframe> tags. It does not work on CSS background-image properties (often used for complex grid layouts or parallax sections).

To lazy load background images efficiently in 2025, use the native Intersection Observer API via Vanilla JavaScript. It runs off the main thread, making it incredibly lightweight compared to traditional scroll-event listeners.

<style>
        .lazy-bg { background-image: none; }
        .lazy-bg.is-visible { background-image: url('heavy-background.webp'); }
    </style>
    
    <script>
    document.addEventListener("DOMContentLoaded", function() {
        let lazyBackgrounds = [].slice.call(document.querySelectorAll(".lazy-bg"));
    
        if ("IntersectionObserver" in window) {
            let lazyBackgroundObserver = new IntersectionObserver(function(entries, observer) {
                entries.forEach(function(entry) {
                    if (entry.isIntersecting) {
                        entry.target.classList.add("is-visible");
                        lazyBackgroundObserver.unobserve(entry.target);
                    }
                });
            });
    
            lazyBackgrounds.forEach(function(lazyBackground) {
                lazyBackgroundObserver.observe(lazyBackground);
            });
        }
    });
    </script>

Conclusion

A solid lazy loading strategy comes down to a few rules: defer off-screen assets, load the LCP hero image eagerly, set explicit width and height to prevent layout shift, and use Intersection Observer for CSS background images. Get those right and you cut initial page weight without hurting the metrics Google actually measures.