What is lazysizes?

lazysizes is a JavaScript library that delays loading images and iframes on a webpage until they are about to come into view, rather than downloading everything at once when the page first opens. This technique is called lazy loading, and it makes pages load faster because the browser skips images the user may never scroll to. The problem it solves is page performance: a page with dozens of images will spend bandwidth downloading images at the bottom that most visitors never see. By deferring those downloads until the user scrolls close to them, the page becomes usable much faster on slower connections. Setup requires almost no configuration. You add a script tag, then change image tags to use a class called "lazyload" and store the real image URL in a data-src attribute instead of the regular src. lazysizes automatically watches for when those elements approach the visible area and swaps in the real URL. It handles responsive images (different sizes served depending on screen width), iframes like embedded videos, and elements that become visible through JavaScript interactions like carousels or infinite scrolling, not just from scrolling the page. A notable feature is automatic calculation of the sizes attribute for responsive images, which tells the browser how large the image will be displayed so it can pick the right resolution. The library is also SEO-friendly: search engine crawlers that do not scroll will see all images immediately. You install it via npm or a script tag. It is written in JavaScript and extends via optional plugins. The full README is longer than what was provided.