• An API providing a way to asynchronously observe changes in the intersection of a target element with an ancestor element or with a top-level document’s viewport. Historically, detecting visibility of an element, or the relative visibility of two elements in relation to each other, has been a difficult task for which solutions have been unreliable and prone to causing browsers and sites to become sluggish. The Intersection Observer API lets code register a callback function that is executed whenever an element they wish to monitor enters or exits another element (or the viewport), or when the amount by which the two intersect changes by a requested amount. ← MDN Web Docs ↑ w3.org/TR/intersection-observer

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