Within Google Search Console (Core Web Vitals), I've received a warning, with 200,000+ webpages, under the message "CLS issue: more than 0.25 (mobile)".

I did some changes, and I managed to obtain a 99% score in virtually all the pages of my website. I checked it with Google PageSpeed Insights, with GTMetrix Performance Report, and with Chrome Lighthouse. In all the cases, the CLS is 0ms. Even I managed to reduce the TTFB up to 200 ms.

I asked for a new validation, but yesterday I received a "Validation failed" message, with 110,000+ URLs "failing", according to GSC. However, if I check those URLs with the above-mentioned tools, I obtain again a 99% score and a CLS=0.

Then I installed the Chrome 'Web Vitals' extension (https://chrome.google.com/webstore/detail/web-vitals/ahfhijdlegdabablpippeagghigmibma), and a few times and just in some of my pages, the square gets red because of CLS, and only when I scroll down and up across the page.

I did include width and height attributes on images, and I did reserve space statically for ad slots. However, I suspect that a floating top banner of Adsense is the generator of the CLS harm.

Any similar experiences? Does anybody know a tool to identify the elements which harm CLS? What can I do? I don't dare to ask for another validation, and be bothering Google.


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