2.Polyfill

IntersectionObserver

https://github.com/w3c/IntersectionObserver

polyfill

https://github.com/w3c/IntersectionObserver/tree/master/polyfill

file

https://github.com/w3c/IntersectionObserver/blob/master/polyfill/intersection-observer.js

cdn.profill.io

https://cdn.polyfill.io/v3/