A great intro by Dan Callahan on why IntersectionObserver is so damn useful:
What do infinite scrolling, lazy loading, and online advertisements all have in common?
They need to know about—and react to—the visibility of elements on a page!
Unfortunately, knowing whether or not an element is visible has traditionally been difficult on the Web. Most solutions listen for scroll and resize events, then use DOM APIs like getBoundingClientRect() to manually calculate where elements are relative to the viewport. This …
IntersectionObserver comes to Firefox is a post from CSS-Tricks