I have been trying to figure out CLS on a site and I have been using the Web Vitals Chrome extension based on Web.dev’s recommendation (https://web.dev/vitals/). However, I can change the width of a test page and get wildly different results. Some widths will get a CLS score of zero. Other widths return a result over .4. As an example, below are some screenshots of the web.dev/cls page showing different results based on width (which I found ironic).
So my question about webpagetest.org’s results is what width do they use for a site? Do they check multiple widths and average the score? How do we get a consistent score to know the site’s true CLS? Better yet, how will Google judge on their search score?
Is there a reliable tool that can tell us what our site truly scores so we can accurately measure and fix sites accordingly? Help, please!