WordPress iframe dimensions and Cumulative Layout Shift

Sites with explicitly sized iframe dimension sizing have a good CLS most often: 86%, against 89% of all WordPress sites. Sites with unsized iframe dimensions have it least often, at 85%.

Unsized is the most used: 82.1% of WordPress sites. Explicitly sized is 62.7% of WordPress sites.

jQuery sets the iframe dimension sizing to explicitly sized on 63% of its sites, and 89% of jQuery sites have a good CLS, against 89% of all sites. Vue sets the iframe dimension sizing to unsized on 85% of its sites, and 89% of Vue sites have a good CLS.

FrameworkIframe dimension sizing on its sitesGood CLS

WordPress sites with iframe dimension sizing 29,444 sitesUnsized iframe (no width/height) 82%Explicit width & height attributes 63%89%

jQuery 28,576 sitesUnsized iframe (no width/height) 82%Explicit width & height attributes 63%89%

Vue 1,361 sitesUnsized iframe (no width/height) 85%Explicit width & height attributes 47%89%

React 1,151 sitesUnsized iframe (no width/height) 87%Explicit width & height attributes 57%85%

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

Does the framework explain the iframe dimension sizing difference?

On jQuery, 86% of sites with explicitly sized iframe dimension sizing have a good CLS, against 85% of jQuery sites with unsized. Within jQuery, sites with explicitly sized and unsized have a good CLS about equally often. On Vue, 85% of sites with explicitly sized iframe dimension sizing have a good CLS, against 84% of Vue sites with unsized. Within Vue, 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, 62.7% size their embeds explicitly (86% good CLS) and 82.1% include unsized iframes (85% good CLS, largely hidden utility sandboxes).