Do these 3 things before closing this tab:
1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsFor ordinary images below the fold, use the browser’s built-in loading="lazy" attribute. Use JavaScript with IntersectionObserver only when you need a custom loading threshold, have a nonstandard target, or need a fallback. Keep images likely to appear in the first viewport—especially the Largest Contentful Paint (LCP) image—eager, because deferring them can delay the page’s most important visual content.
Choose native lazy loading or JavaScript
Native lazy loading is the least complex choice for ordinary offscreen <img> elements. It needs no separate library, works when JavaScript is disabled, and lets the browser decide how far ahead of the viewport to fetch images. Use JavaScript when that browser-controlled threshold is not suitable, when content is not represented by a regular image element, or when your supported browser population needs a fallback.
| Approach | Best for | Control | Tradeoff |
|---|---|---|---|
Native loading="lazy" |
Ordinary offscreen images, including an image inside a <picture> |
The browser chooses when to fetch | Minimal code; threshold is not configurable |
JavaScript with IntersectionObserver |
Custom preload distance, nonstandard targets, or fallback behavior | Set the observed root and rootMargin |
More code and failure cases to handle |
| Scroll and resize handlers | Compatibility fallback where needed | Fully custom | More event-handling and geometry work |
Browsers that do not recognize loading ignore the attribute and load images normally, without deferring them. Feature detection with 'loading' in HTMLImageElement.prototype is available if you have a demonstrated need for a fallback; do not add a library by habit. See web.dev’s browser-level image lazy-loading guidance.
Keep initially visible and LCP images eager
Do not lazy-load an image likely to appear in the initial viewport, particularly the LCP candidate. With lazy loading, the browser waits for layout information before deciding whether to fetch the image. If that image is visible immediately, this can add avoidable resource delay. Keep its URL discoverable in the initial HTML. Normal image loading is eager by default; adding loading="eager" is generally unnecessary unless tooling requires an explicit value. The web.dev Optimize LCP guidance says not to lazy-load the LCP image. If appropriate, fetchpriority="high" can hint that a likely LCP image is important, but use the hint selectively and check the actual resource priority.
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Use native lazy loading for ordinary offscreen images
Add loading="lazy" to images below the fold, while keeping real source URLs in the markup. Include intrinsic dimensions so the browser can reserve the image’s space before the file arrives.
<img src="gallery-01.webp" loading="eager" width="800" height="600" alt="Description">For responsive <picture> markup, place the loading attribute on its fallback <img>. Native thresholds are browser decisions; this attribute does not let you specify a distance from the viewport.
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Implement a custom threshold with IntersectionObserver
Use IntersectionObserver when you need to start loading at a chosen distance before an image enters view. A positive bottom rootMargin expands the observer’s effective area below the viewport. The right distance depends on image placement, viewport, network conditions, and how quickly the asset can load; 0px 0px 256px 0px is an illustrative value, not a universal setting.
This example stores the real URL in data-src, assigns it when the image approaches the viewport, and then stops observing it. It uses native loading when available and falls back to eager source assignment otherwise. Apply it only to deferrable images; leave initial-viewport and LCP images in ordinary HTML with their real src.
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<img class="js-lazy" data-itgw-was-lazy-src="gallery-01.webp" width="800" height="600" alt="Description">
<script>
const images = document.querySelectorAll('img.js-lazy[data-src]');
if ('loading' in HTMLImageElement.prototype) {
images.forEach((img) => {
img.src = img.dataset.src;
img.loading = 'lazy';
img.removeAttribute('data-src');
});
} else if ('IntersectionObserver' in window) {
const observer = new IntersectionObserver((entries, observer) => {
entries.forEach((entry) => {
if (!entry.isIntersecting) return;
const img = entry.target;
img.src = img.dataset.src;
img.removeAttribute('data-src');
observer.unobserve(img);
});
}, { rootMargin: '0px 0px 256px 0px' });
images.forEach((img) => observer.observe(img));
} else {
images.forEach((img) => {
img.src = img.dataset.src;
img.removeAttribute('data-src');
});
}
</script>The native branch above still uses JavaScript to assign the URL from data-src. That pattern is appropriate only when the project intentionally manages sources in JavaScript. For regular images, prefer a real src in the HTML with loading="lazy", which remains available when scripts are disabled and is easier for crawlers to inspect.
Fallback behavior and accessibility
Do not leave meaningful images behind an empty placeholder if JavaScript is unavailable. For content that must be accessible without scripts, keep the image URL in ordinary markup and use the native attribute, or provide a deliberate non-JavaScript fallback for the JavaScript-dependent design. Supply useful alt text for informative images and an empty alt="" for purely decorative images.
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MDN lists an IntersectionObserver polyfill or scroll, resize, and orientation-change handlers as compatibility alternatives. Prefer the observer where available rather than repeatedly measuring every image’s position during scroll events. See MDN’s lazy-loading guide and web.dev’s lazy-loading best practices.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Reserve layout space to prevent shifts
Set image width and height, or reserve an equivalent aspect ratio in CSS, so the layout has the correct space before the image loads. Without dimensions, an image may initially occupy zero space. In a gallery, that can lead the browser to decide that many or all images are in view and fetch them early; when images appear, they can also push other content around. A dimensionally consistent placeholder can reserve space where that fits the design.
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Test the performance effect
Lazy loading is useful when it reduces early work for noncritical images, but it is not automatically faster for every page or image. Browser heuristics, viewport size, network conditions, and image position all affect the result.
- Check representative viewports. Test mobile and desktop sizes because the first viewport contains different content at each size.
- Inspect the network waterfall. Confirm that below-the-fold assets are deferred as intended, visible images appear promptly, and the likely LCP image is not delayed. Check its resource priority if you use
fetchpriority. - Compare performance evidence. Assess lab results and field performance where available; a single viewport or run does not establish a universal improvement.
- Inspect rendered markup and discoverability. Google recommends that lazy-loaded content load when it becomes visible without requiring user interaction. Use Google Search Central’s lazy-loading guidance and URL Inspection Tool to check rendered HTML; verify relevant image URLs appear in
srcattributes.
For large images inserted by JavaScript, decoding may also require main-thread work. HTMLImageElement.decode() is a possible technique, but it has support and complexity tradeoffs and is not a requirement for ordinary small images.
Troubleshoot common lazy-loading problems
- The hero image appears late or LCP worsens: Remove lazy loading from the initially visible or LCP image, keep its URL in initial HTML, and inspect its network priority. Consider
fetchpriority="high"only when appropriate. - Images load immediately instead of being deferred: Check that target images have dimensions or a stable aspect ratio, and verify they are actually below the fold at the tested viewport. Zero-sized images can affect visibility decisions.
- Images never appear: In a JavaScript implementation, inspect console errors and confirm the observer runs, the element is observed, and its real URL is assigned to
srcwhen it intersects. Make sure no content depends on an interaction to trigger loading. - Content jumps when an image arrives: Add intrinsic dimensions or reserve a matching aspect ratio in the layout.
- Search rendering misses images: Check the rendered HTML for the image URL in
src, and confirm the image loads when visible without a click, scroll-triggered interaction, or other user action. - Older browsers do not defer native images: They may ignore
loading="lazy"and load normally. Add a JavaScript fallback only if support requirements justify the extra code.
Or skip the browser setup:
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