Tester Lazy Load Images

Image Lazy Load Tester - Verify Lazy Loading

Image Lazy Load Testerchecks whether the images on a website are using the correct lazy loading. Provide a page URL - the tool will analyze the loading, srcset, IntersectionObserver and other lazy load attributes and display a performance report.

What is lazy loading?

Lazy loading is an optimization technique that loads resources (images, videos, iframes) only when they are visible or close to visible in the browser (viewport). Instead of loading all images at page start, it loads only those "above the fold" (visible without scrolling). Effects: faster first page loading time (FCP, LCP), lower data consumption (important for mobile users), better performance on slow connections, positive impact on Core Web Vitals (Google ranking signals).

Lazy loading implementation methods

Native HTML5 lazy loading- simplest method:<img src="image.jpg" loading="lazy" alt="...">. Supported by all modern browsers (Chrome 76+, Firefox 75+, Safari 15.4+, Edge 79+). No JavaScript needed. The browser automatically decides when to load an image.Intersection Observer API- JavaScript API that detects whether an element is visible in the viewport. Used by libraries such as Lozad.js, Lazysizes, Vanilla LazyLoad.Scroll Events (Deprecated)- Legacy approach, listening for a scroll event. Less effective than Intersection Observer (blocks main thread).srcset and sizes- This is not lazy loading, but responsive loading of different image resolutions depending on the screen size.

Core Web Vitals a lazy loading

Lazy loading has a direct impact on Google metrics:LCP (Largest Contentful Paint)- if the main image of the page (hero image) has loading="lazy", LCP may be delayed! "Above the fold" images should not have lazy loading.CLS (Cumulative Layout Shift)- images without defined dimensions (width/height or aspect-ratio) cause the layout to shift when loaded. Always define the dimensions of your images.FID/INP- Intersection Observer is asynchronous and does not block the main thread - better than scroll events.

Most common lazy loading implementation errors

Bug 1: Lazy loading on hero image (above fold) - delays LCP. Mistake 2: Missing alt attribute - accessibility and SEO issues. Error 3: Missing width/height on images - causes CLS. Mistake 4: Using outdated libraries with scroll events instead of Intersection Observer. Mistake 5: Lazy loading on small icons (SVG) - service cost greater than delay benefit. The tester detects these errors automatically.

Frequently asked questions