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Knowing where the viewport is relative to the document is pretty valuable; It means that AMP can defer loading potentially sluggish content until the viewport lands on a particular position.

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Sounds like a premature optimization to me. The websites I tried did not have such requirements but the scrolling was just as janky nonetheless.

It's not premature optimization. It's optimization for ad revenue; they can charge more if they can prove an advertisement is visible on screen.

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