Probably the best lazy loading implementation available.

• Perfect for responsive images
• Truly lazy (absolutely no unnecessary requests)
• SEO friendly (valid HTML)
• Supports WebP and AVIF
• Customizable load threshold
• Lazy load background images
• Lazy load HTML

You can find a demo at ivopetkov.github.io/responsively-lazy-v3/ and learn how the magic works at ivopetkov.com/responsively-lazy/

Download the minified JS file or install via npm.

npm install responsively-lazy

The library does not have any dependencies, and it's just 2.5kb when gzipped and minified.

• Include the JS file in the head of the document

<script async src="responsively-lazy.min.js"></script>

• Add the following code for each image

<img
    src="images/2500.jpg"
    style="width:100%;aspect-ratio:400/274;"
    srcset="data:image/gif;base64,R0lGODlhAQABAIAAAP///////yH5BAEKAAEALAAAAAABAAEAAAICTAEAOw=="
    data-responsively-lazy="images/400.jpg 400w, images/600.jpg 600w, images/800.jpg 800w, images/1000.jpg 1000w"
/>

Values to customize:

aspect-ratio

The aspect ratio of the image (width/height) to reserve space. Skip if you don't know it.

src

The default (largest) image size. Will be used by very old browsers, search engines, social networks, etc.

data-responsively-lazy (the star of the show)

Must contain a list of image versions separated by commas. Each version must contain a path to the image and its size. Optionaly you can specify the file type (webp or avif). The first supported format for the selected width is used.

data-responsively-lazy="images/400.avif 400w avif, images/400.webp 400w webp, images/400.jpg 400w, ..."

The paths may be encoded if there are special characters like commas, spaces, etc. Example: https%3A%2F%2Fexample.com%2Fimage.jpg

data-responsively-lazy-threshold

Specify how close to the viewport an image should be to start loading.

data-responsively-lazy-threshold="500px" // can use percents too

data-responsively-lazy-type=html

Lazy load HTML. Can contain JavaScript.

<div
    data-responsively-lazy-type="html"
    data-responsively-lazy="hello world"
></div>

data-responsively-lazy-type=background

Lazy load image as a background image.

<div
    data-responsively-lazy-type="background"
    data-responsively-lazy="images/400.jpg 400w, images/600.jpg 600w, images/800.jpg 800w, images/1000.jpg 1000w"
></div>

data-on-responsively-lazy-load

Run JavaScript code when the image is loaded.

data-on-responsively-lazy-load="..."

Responsively Lazy is very different from the other lazy loading libraries. Others make you break your HTML by removing the src attribute, or make you put some tiny version there, or make you use <noscript> to make your images appear in search engines (like Google Images). The following code has worked for ages:

<img src="image.jpg" />

Let's not break it when we can enhance it.

<img src="image.jpg" data-responsively-lazy="image-200.jpg 200w, image-400.jpg 400w" srcset="..." />

The lazy loading works in browsers supporting the srcset attribute and JavaScript. As of December 2023 that's 97.6%. Unsupported browsers, search engines and social networks will load the image in the src attribute. Make sure it's high resolution.

The library will listen for DOM changes and you can also request loading by calling responsivelyLazy.run().

Free to use under the MIT license.

You can find me at @IvoPetkovCom and ivopetkov.com