Iframe dimension sizing and Cumulative Layout Shift
Unsized iframes (81.6%) versus explicitly sized iframes (51.4%): how reserving embed dimensions affects Cumulative Layout Shift across iframe-using sites.
51.4% of sites use explicitly sized iframe dimension sizing, and 85% of those sites have a good CLS, against 88% of all sites.
CLS pass rate
Sites with explicitly sized iframe dimension sizing have a good CLS most often: 85%, against 88% of all sites. Sites with unsized iframe dimensions have it least often, at 85%.
Iframe dimension sizing usage
Unsized is the most used: 81.6% of sites. Explicitly sized is 51.4% of sites.
Understanding iframe dimension sizing
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, 51.4% size their embeds explicitly (85% good CLS) and 81.6% include unsized iframes (85% good CLS, largely hidden utility sandboxes).
Reserve fixed width and height attributes or a CSS aspect-ratio container around every visible iframe so late postMessage resizes do not shift surrounding layout. Across sites with iframes, 51.4% set explicit iframe dimensions (85% good CLS) while 81.6% contain at least one unsized iframe (85% good CLS).
How iframe dimension sizing plays out per platform
What the field data shows when you isolate a single CMS or framework.
WordPress 30,414 sites 62.8% of iframe-using WordPress sites set explicit iframe dimensions (86% good CLS).
Across iframe-using WordPress sites, 62.8% include explicit width and height attributes on iframes (86% good CLS) and 82.1% include unsized iframes (85% good CLS).
Shopify 6,746 stores 97.7% of iframe-using Shopify stores include unsized iframes (93% good CLS).
Across iframe-using Shopify storefronts, 97.7% contain an iframe without HTML width or height attributes (93% good CLS) and 15.8% use explicit dimensions (90% good CLS).
React & Next.js 5,283 sites Iframe-using React sites with sized iframes (62.3%) pass CLS 3 points more often than sites with unsized iframes.
Across 5,283 iframe-using React sites, 62.3% declare explicit width and height on their embeds (92% good CLS) compared with 70.3% with unsized iframes (89% good CLS).
By Arjen Karel · 210,902 crawled sites (196,263 with all devices Chrome field data, 86,714 sites with iframe dimension sizing, October 2026). How we measured.