71. 图像的懒加载如何实现？

图像的懒加载已经有了native support。

Native <img> lazy-loading is coming to the web! https://t.co/LgF7F1iMgR <img loading=lazy> defers offscreen images until the user scrolls near them. Shipping in Chrome ~75 https://t.co/4gR7lvx4zx pic.twitter.com/luCHEfLkKD

— Addy Osmani (@addyosmani) April 7, 2019

如果需要自己实现的话，需要如何做？