What is CLS (Cumulative Layout Shift)?

What to do about it

1. Watch the page load on a slow connection and note what jumps. PageSpeed Insights also lists the elements that shifted.
2. Set width and height attributes (or a CSS aspect-ratio) on every image, video, and embed, so the browser reserves the right space before the file arrives.
3. Reserve fixed space for ads, banners, and late-loading widgets instead of letting them push existing content down.
4. Preload your web fonts so headings do not re-render in a different typeface after the page has painted.
5. Avoid inserting content above existing content after load — late banners above the fold are a classic CLS source.

What WebVitalist checks

The free scan runs 19 technical checks on one public URL. These are the ones that matter for this question:

• HTML size (html-size): None of the free scan checks measure CLS directly. Large HTML documents point at pages where content arrives late and is more likely to shift, so the scan warns when the HTML exceeds 400 kB.
• Server response time (ttfb): A paid audit measures CLS directly, through a Lighthouse run and Chrome UX Report field data. The free scan reports server response time because slow HTML delivery delays everything that follows, including the content that shifts.

A paid audit ($9 and up) also runs a real mobile Lighthouse test through Google PageSpeed Insights and reads field data for LCP, INP and CLS, where Google has it.