Lazyloading in 2025: Load light or load wrong?

It’s 2025. JavaScript powers entire apps, Core Web Vitals still drive SEO requirements, and web accessibility is legally enforced thanks to the European Accessibility Act.

Lazyloading isn’t just a "nice performance trick" anymore, it’s a key lever for performance, SEO, and inclusion. In short: mess it up, and your page falls flat.

So let’s break it down and clear up the confusion.

JavaScript Lazyloading

The Classic JS Pattern (IntersectionObserver) htmlCopierModifier

<img
  src="blank.png"
  data-src="photo.jpg"
  alt="Illustrative image"
  class="lazy"
  width="600"
  height="400"
/>

const observer = new IntersectionObserver(entries => {
  entries.forEach(entry => {
    if (entry.isIntersecting) {
      const img = entry.target;
      img.src = img.dataset.src;
      img.classList.add('loaded');
      observer.unobserve(img);
    }
  });
});

document.querySelectorAll('img.lazy').forEach(img => observer.observe(img));

This approach is flexible — it works on any element (<video>, <iframe>, etc.).
But without HTML fallbacks, the image is invisible to:

• Browsers without JS (yes, they still exist)
• Screen readers expecting an img[src] in the initial DOM
• Google, if rendering is too dynamic or slow

Lazyload indexing in JavaScript

When you lazyload content with  JavaScript, it may not be visible to search engines or browsers without JS. In this case, you use the <noscript> tag. It is essential whenever a visual element is injected or modified via JavaScript.
Example:

<img data-src="photo.jpg" alt="Illustrative image" class="lazy" width="600" height="400" />
<noscript>
  <img src="photo.jpg" alt="Illustrative image" width="600" height="400" />
</noscript>

And to ensure correct display with JS disabled:

<noscript>
  <style>
    .lazy { display: none; }
  </style>
</noscript>

This ensures:

• Google can index content immediately
• Screen readers get a proper, accessible image
• The page remains readable even without JS

If you’re using native lazyloading, though, <noscript> becomes redundant. We’ll get to that...

Native Lazyloading: loading="lazy"

For a few years now, all modern browsers support the loading="lazy" attribute on images, and iframes too.

<img 
  src="photo.jpg" 
  alt="Illustrative image"
  loading="lazy"
  width="600"
  height="400"
/>

<iframe
  src="https://www.youtube.com/embed/abc123"
  loading="lazy"
  width="560"
  height="315"
></iframe>

Pros:

• SEO-friendly: Google sees it all — no need for <noscript> or JavaScript
  
• Accessible by default (as long as alt and dimensions are set)
  
• Fully supported by modern browsers

Cons:

• Not yet supported on <video> (at least, not standardized)

What About Videos? Clean Lazyloading in 2025

Native lazyloading for videos isn't standardized yet. But Google offers a clear method in their guide on video lazyloading: use preload="none" and a poster.

Example:

<video 
  controls
  preload="none"
  poster="poster.jpg"
  width="640"
  height="360"
>
  <source src="video.webm" type="video/webm">
  <source src="video.mp4" type="video/mp4">
</video>

This prevents the browser from preloading the entire video. The poster acts as a lightweight placeholder, and it might even count as your LCP image.

Preload Images Above the Fold!

Images that are visible on initial page load should not be lazyloaded. Instead, they deserve explicit loading priority to avoid being delayed by other resources.
Use <link rel="preload"> for above-the-fold visuals or poster images:

<link rel="preload" href="image.jpg" as="image" fetchpriority="high">

Anything not lazyloaded, but critical for the initial render (LCP, hero image, logo, etc.), should be preloaded.

Bonus: Animate Image Load

HTML

<img
  src="photo.jpg"
  alt="Illustrative image"
  loading="lazy"
  width="600"
  height="400"
/>

CSS

img[loading="lazy"] {
  opacity: 0;
  transition: opacity 300ms ease;
}
img[loading="lazy"].loaded {
  opacity: 1;
}

JavaScript

document.querySelectorAll('img[loading="lazy"]').forEach(img => {
  if (img.complete) {
    img.classList.add('loaded');
  } else {
    img.addEventListener('load', () => {
      img.classList.add('loaded');
    });
  }
});

Fallback <noscript>

<noscript>
  <style>
    img[loading="lazy"] {
      opacity: 1 !important;
    }
  </style>
</noscript>

Result: a smooth fade-in effect when JS is enabled, and instant visibility when it’s not.

TL;DR: Lazyloading Best Practices for 2025

• Lazyload via JS? Use <noscript> for SEO & accessibility
• Using loading="lazy"? No <noscript> needed, it's native
  
• Never lazyload above-the-fold images
  
• Use <link rel="preload"> with fetchpriority="high" for above-the-fold content
  
• Always define width and height to prevent layout shifts (CLS)
  
• Set meaningful alt text (not generated on the fly)
  
• For videos (no autoplay): use preload="none" + poster