Native image lazy-loading for the web

Historically, to limit the impact offscreen images have on page load times, developers have needed to use a JavaScript library (like LazySizes) in order to defer fetching these images until a user scrolls near them. The attribute allows a browser to defer loading offscreen images and iframes until users scroll near them. Browsers that support the attribute can get the new lazy-loading behavior with and those that don’t will still have images load.

Source: addyosmani.com