w3c/intersectionobserver

This repository holds the specification text for Intersection Observer, a web browser feature that lets a webpage's JavaScript find out when a particular element becomes visible on screen, or when it overlaps with another element, without the page having to constantly check scroll position itself. Before this existed, developers had to listen for scroll events and calculate element positions by hand, which is slow and can make a page feel sluggish, especially on things like lazy loading images or infinite scrolling lists. The README is short, since this is a specification repository rather than an application you install and run. It states that Intersection Observer is already built directly into every major browser: Chrome since version 51, Microsoft Edge, Firefox since version 55, and Safari since version 12.1 on Mac and iOS 12.2 on iPhone and iPad. Because support is now this widespread, the JavaScript polyfill that used to live in this same repository for older browsers has been retired and moved to a separate, archived repository. What remains here is the formal specification document itself, along with a separate plain-language explainer document describing the feature's design and reasoning. The spec text is written in a markup language called Bikeshed, which is a tool spec authors use to produce the kind of formatted technical documents the W3C publishes, rather than being a programming language for building software. There is no code to install or configure. Anyone wanting to actually use Intersection Observer in a website simply calls the browser's built in IntersectionObserver API directly from their own JavaScript, since it now needs no external library at all.