It's time to lazy-load offscreen iframes!

Browser-level built-in lazy-loading for iframes is here

Jul 24, 2020 — Updated Nov 29, 2022

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Standardized lazy-loading for images landed in Chrome 76 via the loading attribute and later came to Firefox. We are happy to share that browser-level lazy-loading for iframes is now standardized and is also supported in Chrome and Chromium-based browsers.

<iframe src="https://example.com"
        loading="lazy"
        width="600"
        height="400"></iframe>

Standardized lazy-loading of iframes defers offscreen iframes from being loaded until the user scrolls near them. This saves data, speeds up the loading of other parts of the page, and reduces memory usage.

This demo of <iframe loading=lazy> shows lazy-loading video embeds:

Why should we lazy-load iframes? #

Third-party embeds cover a wide range of use cases, from video players, to social media posts, to ads. Often this content is not immediately visible in the user's viewport. Rather, it's only seen once they scroll further down the page. Despite this, users pay the cost of downloading data and costly JavaScript for each frame, even if they don't scroll to it.

Data-savings from using iframe lazy-loading for an iframe. Eager loading pulls in 3MB in this example, while lazy-loading does not pull in this code until the user scrolls closer to the iframe.

Based off Chrome's research into automatically lazy-loading offscreen iframes for Data Saver users, lazy-loading iframes could lead to 2-3% median data savings, 1-2% First Contentful Paint reductions at the median, and 2% First Input Delay (FID) improvements at the 95th percentile.

Additionally, lazy-loading off-screen iframes can impart benefits to Largest Contentful Paint (LCP). LCP candidates, such as images or text dependent on web fonts in order to render. Because iframes can often require a significant amount of bandwidth in order to load all of their subresources, lazy-loading offscreen iframes can significantly reduce bandwidth contention on network-constrained devices, leaving more bandwidth to load resources which contribute to a page's LCP.

How does built-in lazy-loading for iframes work? #

The loading attribute allows a browser to defer loading offscreen iframes and images until users scroll near them. loading supports two values:

lazy: is a good candidate for lazy-loading.
eager: is not a good candidate for lazy-loading. Load right away.

Using the loading attribute on iframes works as follows:

<!-- Lazy-load the iframe -->
<iframe src="https://example.com"
        loading="lazy"
        width="600"
        height="400"></iframe>

<!-- Eagerly load the iframe -->
<iframe src="https://example.com"
        width="600"
        height="400"></iframe>

Not specifying the attribute at all will have the same impact as explicitly eagerly loading the resource.

If you need to dynamically create iframes via JavaScript, setting iframe.loading = 'lazy' on the element is also supported:

var iframe = document.createElement('iframe');
iframe.src = 'https://example.com';
iframe.loading = 'lazy';
document.body.appendChild(iframe);

iframe-specific lazy-loading behavior #

The loading attribute affects iframes differently than images, depending on whether the iframe is hidden. (Hidden iframes are often used for analytics or communication purposes.) Chrome uses the following criteria to determine whether an iframe is hidden:

The iframe's width and height are 4px or smaller.
display: none or visibility: hidden is applied.
The iframe is placed off-screen using negative X or Y positioning.
This criteria applies to both loading=lazy and loading=auto.

If an iframe meets any of these conditions, Chrome considers it hidden and won't lazy-load it in most cases. iframes that aren't hidden will only load when they're within the load-in distance threshold. Chrome shows a placeholder for lazy-loaded iframes that are still being fetched.

What impact might we see from lazy-loading popular iframe embeds? #

What if we could change the web at large so that lazy-loading offscreen iframes was the default? It would look a little like this:

Lazy-loading YouTube video embeds (saves ~500KB on initial page load):

<iframe src="https://www.youtube.com/embed/YJGCZCaIZkQ"
        loading="lazy"
        width="560"
        height="315"
        frameborder="0"
        allow="accelerometer; autoplay;
        encrypted-media; gyroscope;
        picture-in-picture"
        allowfullscreen></iframe>

Anecdote: when we switched to lazy-loading YouTube embeds for Chrome.com, we saved 10 seconds off of how soon our pages could be interactive on mobile devices. I have opened an internal bug with YouTube to discuss adding loading=lazy to its embed code.

Chrome.com achieved a 10 second reduction in Time To Interactive by lazy-loading offscreen iframes for their YouTube video embed

Lazy-loading Instagram embeds (saves >100KB gzipped on initial load):

Instagram embeds provide a block of markup and a script, which injects an iframe into your page. Lazy-loading this iframe avoids having to load all of the script necessary for the embed. Given such embeds are often displayed below the viewport in most articles, this seems like a reasonable candidate for lazy-loading of their iframe.

Lazy-loading Spotify embeds (saves 514KB on initial load):

<iframe src="https://open.spotify.com/embed/album/1DFixLWuPkv3KT3TnV35m3"
        loading="lazy"
        width="300"
        height="380"
        frameborder="0"
        allowtransparency="true"
        allow="encrypted-media"></iframe>

Although the above embeds illustrate the potential benefits to lazy-loading iframes for media content, there's the potential to also see these benefits for ads.

Facebook's social plugins allow developers to embed Facebook content in their web pages. There's a number of these plugins offered, such as embedded posts, photos, videos, comments… The most popular is the Like plugin - a button that shows a count of who has "liked" the page. By default, embedding the Like plugin in a webpage (using the FB JSSDK) pulls in ~215KB of resources, 197KB of which is JavaScript. In many cases, the plugin may appear at the end of an article or near the end of a page, so loading it eagerly when it's offscreen may be suboptimal.

Thanks to engineer Stoyan Stefanov, all of Facebook's social plugins now support standardized iframe lazy-loading. Developers who opt in to lazy-loading via the plugins' data-lazy configuration will now be able to avoid it loading until the user scrolls nearby. This enables the embed to still fully function for users that need it, while offering data-savings for those who are not scrolling all the way down a page. We are hopeful this is the first of many embeds to explore standardized iframe lazy-loading in production.

Can I lazy-load iframes cross-browser? Yes #

iframe lazy-loading can be applied as a progressive enhancement. Browsers which support loading=lazy on iframes will lazy-load the iframe, while the loading attribute will be safely ignored in browsers which do not support it yet.

It is also possible to lazy-load offscreen iframes using the lazysizes JavaScript library. This may be desirable if you:

require more custom lazy-loading thresholds than what standardized lazy-loading currently offers
wish to offer users a consistent iframe lazy-loading experience across browsers

<script src="lazysizes.min.js" async></script>

<iframe frameborder="0"
	  class="lazyload"
    allowfullscreen=""
    width="600"
    height="400"
    data-src="//www.youtube.com/embed/ZfV-aYdU4uE">
</iframe>

Use the following pattern to feature detect lazy-loading and fetch lazysizes when it's not available:

<iframe frameborder="0"
	  class="lazyload"
    loading="lazy"
    allowfullscreen=""
    width="600"
    height="400"
    data-src="//www.youtube.com/embed/ZfV-aYdU4uE">
</iframe>

<script>
  if ('loading' in HTMLIFrameElement.prototype) {
    const iframes = document.querySelectorAll('iframe[loading="lazy"]');

    iframes.forEach(iframe => {
      iframe.src = iframe.dataset.src;
    });

  } else {
    // Dynamically import the LazySizes library
    const script = document.createElement('script');
    script.src =
      'https://cdnjs.cloudflare.com/ajax/libs/lazysizes/5.2.2/lazysizes.min.js';
    document.body.appendChild(script);
  }

</script>

An option for WordPress users #

You might have many iframes scattered across years worth of post content in a WordPress site. You can optionally add the following code to your WordPress theme's functions.php file to automatically insert loading="lazy" to your existing iframes without having to manually update them each individually.

Note that browser-level support for lazy-loading iframes is also being worked on in WordPress core. The following snippet will check for the relevant flags so that, once WordPress has the functionality built-in, it will no longer manually add the loading="lazy" attribute, ensuring it is interoperable with those changes and will not result in a duplicate attribute.

// TODO: Remove once https://core.trac.wordpress.org/ticket/50756 lands.
function wp_lazy_load_iframes_polyfill( $content ) {
	// If WP core lazy-loads iframes, skip this manual implementation.
	if ( function_exists( 'wp_lazy_loading_enabled' ) && wp_lazy_loading_enabled( 'iframe', 'the_content' ) ) {
		return $content;
	}

	return str_replace( '<iframe ', '<iframe loading="lazy" ', $content );
}
add_filter( 'the_content', 'wp_lazy_load_iframes_polyfill' );

If your WordPress site utilizes caching (hint: it should), don't forget to rebuild your site's cache afterwards.

Conclusion #

Baking in standardized support for lazy-loading iframes makes it significantly easier for you to improve the performance of your web pages. If you have any feedback, please feel free to submit an issue to the Chromium Bug Tracker.

And, in case you missed it, check out web.dev's image and video lazy-loading collection for more lazy-loading ideas.

With thanks to Dom Farolino, Scott Little, Houssein Djirdeh, Simon Pieters, Kayce Basques, Joe Medley and Stoyan Stefanov for their reviews.

Last updated

Nov 29, 2022 — Improve article

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