WordPress iframe dimensions and Cumulative Layout Shift

Sites with unsized iframe dimension sizing have a good CLS most often: 75%, against 78% of all WordPress sites. Sites with explicitly sized iframe dimensions have it least often, at 75%.

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

React sets the iframe dimension sizing to unsized on 87% of its sites, and 75% of React sites have a good CLS, against 78% of all sites.

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%78%

jQuery 28,576 sitesUnsized iframe (no width/height) 82%Explicit width & height attributes 63%77%

React 1,151 sitesUnsized iframe (no width/height) 87%Explicit width & height attributes 57%75%

Vue 1,361 sitesUnsized iframe (no width/height) 85%Explicit width & height attributes 47%75%

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

Does the framework explain the iframe dimension sizing difference?

On jQuery, 75% of sites with unsized iframe dimension sizing have a good CLS, against 75% of jQuery sites with explicitly sized. Within jQuery, sites with unsized and explicitly sized have a good CLS about equally often. On Vue, 72% of sites with unsized iframe dimension sizing have a good CLS, against 71% of Vue sites with explicitly sized. Within Vue, sites with unsized and explicitly sized 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 (75% good CLS) and 82.1% include unsized iframes (75% good CLS, largely hidden utility sandboxes).