Native lazy-loading offscreen iframes
by Marc Radziwill | Oct 14, 2020 | 2 min to read
Lazy loading is a loading strategy that makes it possible to load something later. You may know this from Gatsby, React, or for images. Images or iframes below the fold can load later.
In this post, you read about native lazy-loading iframes. I show you how it works and why you should use it.
Why you should use lazy-loading for iframes?
Third-party embeds vary a lot. You can use an iframe to include video players, social media posts or ads on your website. Often the iframe is not above the fold in the viewport of the user. Not using lazy-loading iframes would cause the browser to download the third-party resources right away.
How does lazy-loading iframe work?
Native lazy-loading an iframe is available via the
loading attribute. You can define three values:
lazy- tell the browser you want to lazy-load it
eager- tell the browser to load it right away
auto- the browser can decide to lazy-load it or not
<!-- Lazy-load the iframe -->
<!-- Eagerly load the iframe -->
<!-- or use loading="eager" to opt out of automatic
lazy-loading in Lite Mode -->
If you don’t use the
loading attribute, it will have the same impact as the
loading="eager". Only in Lite Mode Chrome will use the
auto value to decide how it loads the iframe.
var iframe = document.createElement('iframe');
iframe.src = 'https://example.com';
iframe.loading = 'lazy';
iframe-specific lazy-loading behavior
iframes act a bit differently than images. If an iframe is hidden and meets the following conditions, Chrome won’t lazy-load it. Most websites use hidden iframes for analytics or communication purposes.
Hidden iframe conditions:
- The iframe’s width and height are 4px or smaller.
- display: none or visibility: hidden is applied.
- The iframe is off-screen using negative X or Y positioning.
What is the impact on lazy-loading iframes?
The web.dev team collected some case studies with remarkable results.
- Lazy-loading YouTube video embeds (saves ~500KB on initial page load)
Lazy-loading Instagram embeds (saves >100KB gzipped on initial load)
Lazy-loading Spotify embeds (saves 514KB on initial load)
Native support for lazy-loading iframes makes it easier to improve the performance of websites. The above examples show the impact of media embeds, but you could imagine there is a massive potential for ads. In this post, you read about native lazy-loading iframes. I showed you how it works and why you should use it.