Iframe dimension sizing on React sites

Sites with explicitly sized iframe dimension sizing have a good CLS most often: 92%, against 92% of all React sites. Sites with unsized iframe dimensions have it least often, at 89%.

Unsized is the most used: 70.3% of React sites. Explicitly sized is 62.3% of React sites.

Wix sets the iframe dimension sizing to explicitly sized on 65% of its sites, and 97% of Wix sites have a good CLS, against 92% of all sites. WordPress sets the iframe dimension sizing to unsized on 87% of its sites, and 88% of WordPress sites have a good CLS.

PlatformIframe dimension sizing on its sitesGood CLS

React sites with iframe dimension sizing 5,283 sitesUnsized iframe (no width/height) 70%Explicit width & height attributes 62%92%

Wix 3,352 sitesExplicit width & height attributes 65%Unsized iframe (no width/height) 61%97%

WordPress 1,198 sitesUnsized iframe (no width/height) 87%Explicit width & height attributes 57%88%

Unsized iframe (no width/height) Explicit width & height attributes

Does the platform explain the iframe dimension sizing difference?

On Wix, 97% of sites with explicitly sized iframe dimension sizing have a good CLS, against 95% of Wix sites with unsized. Within Wix, sites with explicitly sized and unsized have a good CLS about equally often. On WordPress, 85% of sites with explicitly sized iframe dimension sizing have a good CLS, against 86% of WordPress sites with unsized. Within WordPress, sites with explicitly sized and unsized have a good CLS about equally often.

Why visible iframes cannot infer an aspect ratio without attributes

Unlike images, an <iframe> has no intrinsic dimensions in its network header; without explicit width and height attributes or a sized container, the browser defaults the frame to 300x150 pixels until scripts resize it. Across iframe-using sites, 62.3% size their embeds explicitly (92% good CLS) and 70.3% include unsized iframes (89% good CLS, largely hidden utility sandboxes).