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Discussion on: Measuring Cumulative Layout Shift (CLS) in WebPageTest

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Rick Viscomi Author

This is a great question and one of the challenges of measuring CLS in the lab.

Field data is inclusive of all of the actions users take on the website like scrolling and interacting with buttons. CLS may be occurring in these kinds of scenarios that lab tools like WebPageTest and Lighthouse don't currently simulate by default. With CLS gaining popularity as a Core Web Vital metric, I'd love to see lab tools incorporate more user-like behaviors to be comparable with field data.