Here's what I'd like you to know upfront: this is a hard problem. If you've landed here because you're hoping to be pointed at a tool you can run that tells you exactly what CSS you can delete from your project, well... there are tools out there, but I'm warning you to be very careful with them because none of them can ever tell you the complete story. Post written by Chris Coyer.

LazyLoad is a fast, lightweight and flexible script that speeds up your web application by loading images, video or iframes as they enter the viewport. It’s written in plain “vanilla” JavaScript, uses Intersection Observers, and supports responsive images. It’s also SEO-friendly and it has some other notable features.

Philip Walton published a new article where he do a deep dive into improving website performance with a technique he's calling Idle Until Urgent.