https://github.com/verlok/vanilla-lazyload   
https://github.com/ApoorvSaxena/lozad.js
https://github.com/malchata/yall.js
https://github.com/ressio/lazy-load-xt
https://github.com/aFarkas/lazysizes

https://lazy-loading.firebaseapp.com/lazy_loading_lib.html