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More can be found in google the Google support document

How to measure

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While this is the most extreme example, it shows how important it is to reserve this space. In the previous example, we can see the exact same ad on the same page having very different results before and after this small optimization.

What height should I reserve?

For ads with a fixed height, it's easy we need to reserve the exact height of the ads.

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If you don’t know the height of ads used please consult with your account manager.

What about the width? Should we also limit allocate it?

With the exception of very particular cases, it's very rare to have horizontal expansion derived from the ads, so usually, there is no need to allocate any space based on the width.

How does asynchronous and lazy loading affect CLS?

Loading the ads while the user scrolls down and reads the contents on the page, increases the ads viewability, but it also increases the chance of having page elements being pushed while visible, which will increase the CLS. For that reason, it's almost mandatory to apply the previous techniques in all the ads that can push any content in the page, to assure the best possible optimization.

How does setting a height of a placement affects parallax?

If we try to set a fixed height for an ad that will be "parallaxing", we may prevent the solution to work as intended if the wrong height is set. The solution is to only reserve the min-height, it's still recommended to review this with your account manager.

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