71. How to implement lazy load for images?

There is native lazy-load for images now on the web.

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

How would you implement lazy-load before this ?