Mozilla Image Lazy Loader

mozilla-lazy-load.js is a JavaScript library that will lazy load <img> elements on a web page as the user scrolls the viewport. It requires support for Intersection Observer API, and will fallback to loading all images on page load in non-supporting browsers.


Each image to be lazy loaded should match the following HTML structure:

<div class="lazy-image-container">
    <img class="lazy-image" src="path/to/placeholder.png" data-src="path/to/image.png" data-srcset="path/to/image-high-res.png 2x" alt="">
        <img class="lazy-image" src="path/to/image.png" srcset="path/to/image-high-res.png 2x" alt="">

This above HTML snippet is also available via a handy Python helper that can be used directly in any bedrock template:

{{ lazy_img(image_url='path/to/image.png', placeholder_url='path/to/placeholder.png', include_highres_image=True, optional_attributes={'class': 'some-class-name'}) }}


  • image_url - Path to final image src.

  • placeholder_url - Path to placeholder image for use before final image is loaded. This image should be small in file size, and can be shared between all images on a page that are to be lazy loaded.

  • include_highres_image - A boolean value to indicate a high resolution version of an image is available. When True, the function will automatically look for the image in the image_url parameter suffixed with ‘-high-res’ and switch to it if the display has high pixel density. Defaults to False.

  • optional_attributes - A dictionary of custom HTML attributes you may want to add to an image.

To initialize the lazy loading plugin, in your JS run:



If you don’t want to use the Python helper provided above and instead use your own HTML, you can pass an alternate custom CSS selecter:

<div class="my-own-lazy-loading-container">
    <img src="path/to/placeholder.png" data-src="path/to/image.png" data-srcset="path/to/image-high-res.png 2x" alt="">
        <img src="path/to/image.png" alt="">


This will instantiate the plugin to observe intersection of elements using the DOM elements matching the selector provided.

Fading in images

By default the lazy load plugin will simply swap in images as they load. To add a little extra visual flourish, you can add a custom fade-in effect using CSS:

.lazy-image-container img {
    display: block;
    opacity: 1;
    transition: opacity 0.3s;

.lazy-image-container img[data-src] {
    opacity: 0;
    display: none;

This works because the plugin removes the data-src attribute as each image lazy loads.

These styles are also available in /media/css/base/mozilla-lazy-load.scss if you want to include that in the static bundle for the page.