What is intersectionobserver?

This repo is the home for the Intersection Observer, a web browser feature that lets a website efficiently detect when an element on the page scrolls into (or out of) view. The most common everyday use is "infinite scrolling" feeds, where new content loads automatically as you read toward the bottom of a page. It is also handy for loading images only when they are about to be seen, or pausing a video the moment it scrolls off-screen. Before this feature existed, developers had to resort to hacky workarounds, constantly running scroll calculations and measuring element positions, which could make pages feel janky and drain battery life on laptops and phones. The Intersection Observer solves this by letting developers hand that tracking work over to the browser engine itself, which can do it far more efficiently at a native level. Instead of the website asking "is it visible yet?" dozens of times per second, the developer simply says "tell me when this element enters the viewport," and the browser sends a single callback when it happens. This repository serves as the original explainer and specification hub for that browser capability. It contains a document explaining the motivation and design behind the feature, a link to the draft specification, and a polyfill, a piece of JavaScript that simulates the feature for older browsers that don't support it natively. The implementation status shows that it has been widely supported across major browsers for years, meaning most developers can now rely on it without a fallback. Web developers building content-heavy sites, news feeds, image galleries, social media timelines, or ad dashboards, are the primary audience. Anyone who has watched a long page stutter while scrolling because of heavy background processing will appreciate the difference this makes. By making visibility detection a built-in browser task, pages stay smooth and responsive even when they are packed with media.